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We generated a novel strain of mutant mouse with a deletion in the gene encoding metabotropic glutamate receptor 1 (mGluR1). Gross anatomy of the hippocampus, excitatory synaptic transmission, long-term depression, and short-term potentiation in the hippocampal CA1 region are all apparently normal in the mutant mice. In contrast, long-term potentiation (LTP) is substantially reduced, and a moderate level of impairment is observed in context-specific associative learning. We propose that mGluR1 is not "in line" in LTP production, but rather modulates the plasticity process, and hence affects context-specific associative learning.

Mice deficient for the gene encoding alpha-calcium-calmodulin-dependent kinase II (alpha-CaMKII knockout mice) provide a promising tool to link behavioral and cellular abnormalities with a specific molecular lesion. The heterozygous mouse exhibited a well-circumscribed syndrome of behavioral abnormalities, consisting primarily of a decreased fear response and an increase in defensive aggression, in the absence of any measured cognitive deficits. Unlike the heterozygote, the homozygote displayed abnormal behavior in all paradigms tested. At the cellular level, both extracellular and whole-cell patch clamp recordings indicated that serotonin release in putative serotonergic neurons of the dorsal raphe was reduced. Thus, alpha-CaMKII knockout mice, in particular the heterozygote, may provide a model for studying the molecular and cellular basis underlying emotional disorders involving fear and aggression.

MHC class I molecules present peptides generated by processing of endogenously synthesized proteins to CD8+ T lymphocytes. Recently, large proteolytic complexes, termed proteasomes, were implicated in antigen processing. Two proteasomal subunits, LMP2 and LMP7, are encoded within the MHC class II region, but their precise role in antigen processing is unknown. We have generated mice that harbor a disruption in their LMP2 gene. Proteasomes purified from spleen and liver of these mutant mice exhibit altered peptidase activities, and antigen-presenting cells showed reduced capacity to stimulate a T cell hybridoma specific for H-2Db plus a nucleoprotein epitope of an influenza A virus. The mutant mice have reduced (60%-70% of wild type) levels of CD8+ T lymphocytes and generate 5- to 6-fold fewer influenza nucleoprotein-specific cytotoxic T lymphocyte precursors. These findings indicate that LMP2 influences antigen processing.

We have generated TCR transgenic mice (T/R+) specific for myelin basic protein (MBP) and crossed them to RAG-1-deficient mice to obtain mice (T/R-) that have T cells expressing the transgenic TCR but no other lymphocytes. Both T/R+ and T/R- mice carry, in the lymph nodes and spleen, large numbers of the potentially encephalitogenic CD4+ anti-MBP T cells. These cells respond to MBP in vitro but show no signs of activation in vivo. Nevertheless, approximately 14% of H-2u T/R+ and 100% of H-2u T/R- mice developed spontaneous experimental autoimmune encephalomyelitis (EAE) within 12 months. These data indicate that EAE can be mediated by CD4+ anti-MBP T cells in the absence of any other lymphocytes and that nontransgenic lymphocytes that are present in T/R+ but absent in T/R- mice have a protective effect. The data also suggest that spontaneous EAE may be triggered by an in situ activation of CD4+ anti-MBP cells in the nervous system.

We describe the development and function of the peripheral lymphoid system of mutant mice rendered deficient in either alpha beta or gamma delta T cells via targeting of TCR genes in embryonic stem cells. In the spleen of alpha beta T cell-deficient mice, gamma delta T cells do not compensate in numbers for the lack of alpha beta T cells, but B cells do. alpha beta T cell-deficient mice are unable to mount an antibody response to ovalbumin and do not reject skin allografts. Natural killer cell function is not impaired in any of the mutant mice. TCR mutant mice will prove useful in dissecting differential functions of alpha beta and gamma delta T cells in vivo.

Expression of the T cell receptor beta (TCR beta) chain is necessary for the transition from the CD4CD8- stage in the major alpha beta thymocyte lineage. The protein tyrosine kinase p56lck has been implicated in the regulation of early thymocyte differentiation and of allelic exclusion at the TCR beta locus. Using mice overexpressing an activated lck transgene and mice with a disruption of the lck gene, we demonstrate that p56lck participates in a pathway that regulates the expansion of the pool of CD4+CD8+ thymocytes to wild-type levels. In addition, p56lck may be involved in the down-regulation of the putative pre-TCR on CD4+CD8+ thymocytes.

Sensory pathways of the brain generally develop from crudely wired networks to precisely organized systems. Several studies have implicated neural activity-dependent mechanisms, including N-methyl-D-aspartate (NMDA) receptors, in this refinement process. We applied the gene targeting to the NMDAR1 gene and created a mutant mouse that lacks functional NMDA receptors. The development of whisker-related patterns in the trigeminal nuclei of the mutant mice and their normal littermates was compared. We show that in the mutant mice pathfinding, initial targeting, and crude topographic projection of trigeminal axons in the brainstem are unaffected, but that whisker-specific patches fail to form. Our results provide a direct demonstration of the involvement of the NMDA receptor in the formation of periphery-related neural patterns in the mammalian brain.

The transporter associated with antigen processing (TAP) delivers peptides to the lumen of the endoplasmic reticulum in an adenosine triphosphate (ATP) dependent fashion for presentation by major histocompatibility complex class I molecules. We show that the mouse TAP translocator (H-2b haplotype) selects peptides based on a minimal size of nine residues, and on the presence of a hydrophobic COOH-terminal amino acid. The preponderance of COOH-terminal hydrophobic amino acids in peptides capable of binding to mouse class I molecules thus fits remarkably well with the specificity of the TAP translocator. In addition to transport in the lumenal direction, efflux of peptide in the cytosolic direction is observed in an ATP- and temperature-dependent manner. By maintaining a low peptide concentration at the site of class I assembly, this efflux mechanism may ensure that class I molecules are loaded preferentially with high affinity peptides.

The functional role of gamma delta T cells (expressing the gamma delta heterodimeric T-cell receptor for antigen) in infectious diseases remains largely unknown. We have therefore attempted to define the possible role of these T cells in the immune response against the various developmental stages of malaria parasites. For this purpose, we monitored the immune response and the development of liver and blood stages of Plasmodium yoelii, a rodent malaria parasite, in immunized and nonimmunized alpha beta T-cell-deficient and gamma delta T-cell-deficient mice. Immunization of alpha beta T-cell-deficient mice with irradiated sporozoites induced an immune response that significantly inhibited the development of the parasite's liver stages. This inhibitory immune response was abolished by an antibody-mediated transient in vivo depletion of gamma delta T cells. Two gamma delta T-cell clones were derived from malaria-immunized alpha beta T-cell-deficient mice. The adoptive transfer of one of these gamma delta T-cell clones to normal mice inhibited the development of liver stages, following sporozoite inoculation. These results provide evidence for gamma delta T-cell-mediated protective immunity against parasites, in the absence of alpha beta T cells. As for the blood phase of the infection, both normal mice and gamma delta T-cell-deficient mice cleared the blood stages of the nonlethal strain of P. yoelii, while alpha beta T-cell-deficient mice failed to control the parasitemia.

Publisher Summary Using gene targeting in embryonic stem cells, mice with mutations in T cell receptor (TCR) gene α, TCRβ, TCRδ, or the recombination activating gene 1 (RAG-1) have been created. TCRα and TCRβ mutant mice are deficient in αβ T cells, but still contain γδ T cells and B cells; TCRδ mutant mice are deficient in γδ T cells, but still contain αβ T cells and B cells. Mice that are doubly mutant for TCRβ and TCRδ do not have any T cells, but still have B cells. However, RAG-1 mutant mice are totally deficient in T cells and B cells. This chapter focuses on thymocyte development and on the functional analysis of these mutant mice that are selectively deficient in either αβ or γδ T cells, with the aim of determining differential functions of αβ and γδ T cells in vivo. The TCR mutant mice are useful models that help to obtain more insights into the differential roles of αβ and γδ T cells in vivo.

We have analyzed the cytolytic activity of freshly isolated intraepithelial T cells (i-IEL) from the intestines of several different mouse strains in an anti-T-cell receptor monoclonal antibody-mediated redirected lysis assay. The cytolytic activity of gamma delta i-IEL but not that of alpha beta i-IEL was strain dependent. Mouse strains could be divided into high (H), marginal (M), and null (N) strains. The anti-gamma delta T-cell receptor monoclonal antibody-induced interferon gamma production showed the same strain-dependent variability, but the proliferative responses to gamma delta T-cell receptor crosslinking did not show this variability. The N phenotype of gamma delta i-IEL was found to be dominant in (H x N)F1 mice. In radiation bone-marrow chimeras the H/N phenotype was determined by the genotype of the reconstituting bone-marrow-derived cells but was not determined by the genotype of the radioresistant host cells. Analysis of (H x N)F1 backcross animals indicated that at least two genes are involved in determination of the H/N phenotype. One of these genes is major-histocompatibility-complex linked. No difference in the use of the variable region segment of the gamma-chain or delta-chain was seen between the gamma delta i-IEL from H and N strains. Various models that might explain the strain-dependent gamma delta i-IEL phenotypes are discussed.

The V(D)J recombination activation gene RAG-1 was isolated on the basis of its ability to activate V(D)J recombination on an artificial substrate in fibroblasts. This property and the expression pattern in tissues and cell lines indicate that RAG-1 either activates or catalyzes the V(D)J recombination reaction of immunoglobulin and T cell receptor genes. We here describe the introduction of a mutation in RAG-1 into the germline of mice via gene targeting in embryonic stem cells. RAG-1-deficient mice have small lymphoid organs that do not contain mature B and T lymphocytes. The arrest of B and T cell differentiation occurs at an early stage and correlates with the inability to perform V(D)J recombination. The immune system of the RAG-1 mutant mice can be described as that of nonleaky scid mice. Although RAG-1 expression has been reported in the central nervous system of the mouse, no obvious neuroanatomical or behavioral abnormalities have been found in the RAG-1-deficient mice.

The gamma delta T cell receptor (TCR) of the hybridoma KN6 recognizes the self molecule encoded by a class I gene which maps within the TL region of the major histocompatibility complex (MHC) of H-2b mice. Mice transgenic (Tg) for this TCR were crossed with mice genetically deficient in beta 2-microglobulin (beta 2m). No mature Tg gamma delta T cells were detected in the thymus or the spleen of the beta 2m- gamma delta Tg mice. We conclude that interaction between the Tg gamma delta TCR and a beta 2m-associated molecule (probably an MHC class I molecule) is required for the generation of mature Tg gamma delta T cells.

T-cell antigen receptor (TCR) heterodimers of both the alpha beta and gamma delta types are expressed at the surface of T cells only in association with a complex of invariant chains called CD3. The requirement for individual CD3 components to achieve TCR surface expression was examined by cotransfection of a non-T-cell line with TCR alpha and beta, as well as CD3 delta, epsilon, gamma, and zeta, cDNAs. Both transient and stable transfectants expressing TCR and CD3 epitopes at the cell surface were generated. By transfection of TCR and CD3 components in different combinations, the TCR chains, as well as the CD3 epsilon and zeta chains, were each shown to be essential for reconstituting surface expression. On the other hand, CD3 delta and gamma chains could be used alternatively, providing evidence for two different types of TCR/CD3 complexes.

Recently it has become possible to introduce predesigned mutations into a given gene in the mouse germ line by homologous recombination in embryonic stem cells. The mutations are usually introduced by inserting the neomycin phosphotransferase gene into an exon of a particular gene. Here we describe an extension of this method that can result in at least a 15-kilobase-long deletion. The deletion created in the present work encompasses one of the two diversity gene segments of the mouse T-cell receptor beta-subunit locus, 10 out of the 12 joining gene segments, and both constant gene segments. This strategy is a valuable alternative to sequential targeting of multiple genes forming a gene cluster, could simplify the construction of plasmids to be used for targeting, and could be the solution for inactivating small genes that have eluded conventional targeting approaches.

We have studied the ligand specificity of a gamma delta T-cell receptor (TCR) derived from a mouse T-cell hybridoma (KN6). KN6 cells reacted with syngeneic (C57BL/6) cells from various origins (splenocytes, thymocytes, peritoneal exudate cells, etc.) and cells from many different mouse strains. KN6 reactivity against cells from a panel of congenic and recombinant mouse strains demonstrated that the ligand recognized by KN6 is controlled by an MHC-linked gene that most probably maps in the TL region. We cloned this gene and formally proved that it does map in the TL region. This gene turned out to be a novel class I gene (designated T22b) belonging to a hitherto unidentified cluster of TL region genes in strain C57BL/6. This gene was expressed in many different tissues and cell types. We also examined the tissue expression of several other TL genes. One of these, the structural gene (T3b) encoding the thymus leukemia (TL) antigen from C57BL/6 mice, was specifically expressed in the epithelium of the small intestine. Since the intestinal epithelium of the mouse is known to be the homing site for a subset of gamma delta T cells (i-IEL) bearing diverse TCR with V7 rearranged gamma chains, we propose that the T3b gene product is part of the ligand recognized by some of the i-IEL. Our data support the idea that gamma delta T cells might be specific for non-classical class I or class I-like molecules and suggest that gamma delta TCR and non-classical MHC co-evolved for the recognition of a conserved set of endogenous or foreign peptides.

The gamma delta and alpha beta T-cell antigen receptor (TCR) heterodimers are expressed in a lineage-specific, mutually exclusive manner. Regulation of expression occurs at the transcriptional level. A 13-kilobase (kb) stretch of DNA encompassing variable-joining-constant segments V gamma 4-J gamma 1-C gamma 1 of the murine gamma-chain gene was examined for the presence of transcriptional enhancing elements by a transient transfection assay. DNA fragments from this region were inserted into a test plasmid containing a heterologous promoter fused to the human growth hormone gene. An 1800-base-pair (bp) fragment located 3 kb 3' to C gamma exon III was found to display enhancing activity in several T-cell lines. Maximum enhancing activity could be localized further to fragments as small as 400 bp in some cell lines. Nucleotide sequence analysis of this 400-bp segment revealed homologies to previously described core enhancer elements and to other TCR gene enhancers. The TCR gamma-chain gene enhancer is active in both gamma delta and alpha beta T cells, indicating that it is not primarily responsible for lineage-specificity of expression, but it is inactive in non-T-cells.

A T lymphocyte expresses on its surface one of two types of antigen receptor, T-cell receptor alpha beta or T-cell receptor gamma delta, encoded by a pair of somatically rearranged alpha and beta or gamma and delta genes. It has been suggested that alpha beta T cells are generated only from precursor T cells that failed to rearrange gamma and delta genes in a functional form. However, we found that transgenic mice constructed with functionally rearranged gamma and delta genes produce a normal number of alpha beta T cells. The transgene gamma present in these alpha beta T cells is repressed apparently through an associated cis DNA element (silencer). We propose that some T-cell precursors are committed to generate alpha beta T cells independent of the rearrangement status of their gamma gene and that this commitment involves activation of a factor(s) that interacts with the gamma gene-associated silencer.

The H-2 nomenclature was last revised in 1974 (Klein et al. 1974; Shreffler et al. 1974). Since then it has gradually gone out of step with the advances in molecular biology of the H-2 complex as well as the rules for genetic nomenclature established by the International Committee for Mouse Genetic Nomenclature (Lyon 1989). Furthermore, multiple symbols are currently in use designating the same H-2 genes or their products. The present proposal rectifies the existing inconsistencies in the H-2 nomenclature while preserving as much as possible from the previous proposals.

Brain-derived neurotrophic factor (BDNF) is a candidate molecule for regulating activity-dependent synaptic plasticity on the grounds of its expression pattern in developing visual cortex and that of its receptor, trkB (Castr?n et al., 1992; Bozzi et al., 1995; Schoups et al., 1995; Cabelli et al., 1996), as well as the modulation of these patterns by activity (Castr?n et al., 1992; Bozzi et al., 1995; Schoups et al., 1995). Infusing trkB ligands or their neutralizing agents, the trkB-IgG fusion proteins, into visual cortex alters the development and plasticity of ocular dominance columns (Cabelli et al., 1995; Riddle et al., 1995; Galuske et al., 1996 ; Gillespie et al., 1996; Cabelli et al., 1997). To test further the physiological role of BDNF, we studied a transgenic mouse that expresses elevated levels of BDNF in primary visual cortex (V1) postnatally (Huang et al., 1999). We found that unlike the infusion experiments, excess BDNF expressed in mouse visual cortex did not block ocular dominance plasticity. Instead, single neurons in V1 of the BDNF transgenic mice were as susceptible to the effects of monocular deprivation (MD) as neurons in wild-type mice, but only during a precocious critical period. At a time when V1 in the wild-type mouse responded maximally to a 4 d MD with a reduction in its response to deprived eye visual stimulation, the transgenic mouse V1 had already passed the peak of its precocious critical period and no longer responded maximally. This finding suggests a role for BDNF in promoting the postnatal maturation of cortical circuitry.

Last month, 70 members of the U.S. Congress, including Henry Hyde, Chairman of the House Judiciary Committee, and J. C. Watts Jr. Republican Conference Chairman, signed a letter urging the federal government to ban all research on stem cells obtained from human embryos and fetuses. The letter calls upon the U.S. Department of Health and Human Services (DHHS) to reverse National Institutes of Health (NIH) Director Harold Varmus's decision to allow funding of pluripotent stem cell research. The lawmakers object “in the strongest possible terms” to Varmus's decision, as well as to the memorandum issued in January by DHHS General Counsel Harriet Rabb, which served as the legal basis for Varmus's position. In their letter, the members of Congress state, “Any NIH action to initiate funding of such research would violate both the letter and spirit of the federal law banning federal support for research in which human embryos are harmed or destroyed.” Federal laws and regulations, they claim, have protected human embryos and fetuses “from harmful experimentation at the hands of the Federal government” for more than two decades. “This area of law has provided a bulwark against government's misuse and exploitation of human beings in the name of medical progress. It would he a travesty for this Administration to attempt to unravel this accepted ethical standard.”

The translocated c-myc gene in AW-Ramos, a Burkitt lymphoma cell line carrying the 8;14 translocation, is expressed at 2- to 5-fold higher levels than c-myc in lymphoblastoid cell lines. The translocation event has joined c-myc to the IgM switch region. As a consequence, a recently identified immunoglobulin transcriptional enhancer element is not linked to the translocated c-myc gene. Chromosomal recombination occurs approximately equal to 340 nucleotides upstream of the c-myc 5' cap site, leaving all three c-myc exons intact. The nucleotide sequences of the two coding exons in the translocated c-myc gene are identical to those of the normal c-myc gene. Nucleotide sequence analyses of the first, noncoding c-myc exon and of the region between this exon and the chromosomal recombination point reveal two single-base differences from normal c-myc. Our data indicate that altered expression rather than an altered gene product is responsible for c-myc activation in AW-Ramos cells and that this is a result of either loss of regulatory sequences located greater than 340 nucleotides upstream of c-myc or disruption of normal c-myc regulation by one or both base substitutions. Alternatively, unidentified enhancer-like sequences in the Ig locus may alter the expression of c-myc.

A 4.8 kilobase mouse embryo DNA fragment was inserted into a phage lambda genome and was subsequently characterized by electron microscopy, restriction enzyme mapping and partial DNA sequencing. This fragment contains a 400 base sequence which is homologous to that of an immunoglobulin light lambda chain mRNA which spans 3.3 to 3.7 kilobases from one end of the fragment. Restriction enzyme mapping as well as partial nucleotide sequencing of the 3' terminal of the homology region confirm the previous conclusion [Tonegawa, Brack, Hozumi and Schuller, Proc. Natl. Acad. Sci. USA. 74, 3518-3522 (1977)] that the cloned DNA fragment contains a Vlambda gene sequence which is separate from any Clambda sequence.

To explore the possibility that the difference in antigen recognition between B and T cells derives from a structural difference in their respective antigen-specific receptors (immunoglobulins on B cells and immunoglobulin-like molecules on T cells), we compared the extracellular segments of the T-cell receptor alpha, beta, and gamma polypeptide chains and the N-terminal segment of the T-cell T8 (Lyt-2) antigen chain with the corresponding regions of immunoglobulins whose three-dimensional structures are known. The results indicate that the four T-cell polypeptide chains are organized into immunoglobulin-like domains consisting of multistranded antiparallel beta-sheet bilayers. Invariant amino acid side chains that are conserved in diverse immunoglobulins, including those that mediate domain-domain interactions and form a constant scaffold for antibody binding sites, are also conserved in the chains encoded by the T-cell receptor genes and in the N-terminal domain of T8 (Lyt-2). It appears that the binding sites of the antigen-specific T-cell alpha beta-chain receptors and of antibodies are very similar in their overall dimensions and geometry: a T-cell alpha beta receptor molecule probably has an antigen-specific binding site that is fundamentally no different than the conventional binding site of an antibody.

The molecular basis for the dependence of antigen recognition by T cells on products of the major histocompatibility complex (MHC) is unknown, and the antigenic structures that are actually bound by T-cell receptors are ill-defined. In this study, we asked whether a monoclonal antibody (mAb) that reacts with the T-cell receptor of a clone of murine cytotoxic T lymphocytes (CTL) and not with the receptors of other CTL clones can substitute for that clone's natural ligand in specific cytolytic reactions. To answer the question, a mAb (1B2) to the receptor of a CTL clone (2C) was attached covalently to 51Cr-labeled cells that were not otherwise susceptible to lysis by clone 2C, and the cells thus modified were then tested as targets for clone 2C and other CTL clones of similar specificity. All labeled cells modified in this way, including a murine cell line that expresses no cell-surface MHC class I molecules and a human cell line, were lysed by clone 2C but not by other CTL clones. If, however, instead of attaching the mAb to the receptor of clone 2C, the cells were modified by attaching to them mAbs to other surface antigens on CTL [lymphocyte function-associated antigen (LFA-1), Thy-1.2], they were not lysed. In cytolytic titrations, the cells that had been converted by attachment of mAb 1B2 into specific targets for clone 2C were just as susceptible to lysis by that clone as the clone's natural H-2d targets (e.g., P815 cells). However, some accessory surface molecules (LFA-1, Lyt-2) that are required for clone 2C to lyse its natural H-2d targets seemed not to be required for this clone to lyse the mAb-converted target cells. By demonstrating that a variety of different cell types can be thus converted into target cells for CTL, the approach described in this study may provide opportunities to analyze further the mechanisms by which CTL destroy target cells.

The search for the genes encoding the T-cell receptor alpha and chains revealed a third gene, T gamma (ref. 1), which shares with t T alpha (refs 2-7) and T beta (refs 8-15) genes a number of structure features, including somatic rearrangement during T-cell development. T gamma gene expression appears to be unnecessary in son mature T cells and is at its greatest in fetal thymocytes encouraging speculation that T gamma has a role in T-cell development and may be involved in the recognition of polymorphic major histocompatibility complex (MHC) products during thymic education. One argument against the participation of T gamma in such a process has been its apparently limited diversity, due to the small number of gene segments available for rearrangement. We here describe the identification of additional T gamma V-gene segments and demonstrate that they can be rearranged to previously identified J- and C-gene segments and are expressed in fetal thymocytes. In addition we describe a variety of patterns of T gamma mRNA processing which may be significant for T gamma gene regulation.

RNA preparations containing 70-80% mouse κ-chain mRNA have been prepared. The remainder consists of many RNA species, each of which represents a small fraction of the total RNA. The κ-chain mRNA preparation hybridizes with mouse liver DNA with bi-phasic kinetics, indicating that it consists of two fractions —“unique” and “reiterated.” Competition hybridization experiments show that the homology among the unique fractions from different mRNAs is the same as the homology among the amino acid sequences of the corresponding κ-chains. Hence, in addition to the C-region (constant-region) sequences, (most of) the V-region (variable-region) sequences are also derived from unique germ line genes. The reiterated fractions from different κ-chain mRNAs show essentially complete homology with each other. This fraction seems to consist mostly of sequences which do not code for amino-acid sequences of the secreted polypeptide chain, i.e., the “external” section of the mRNA molecule. It is concluded that the number of germ line genes is too small to account for the observed diversity of antibody molecules.

En plus des deux genes prealablement identifies et exprimes dans un clone de lymphocytes T cytotoxiques, on identifie un troisieme gene egalement rearrange et exprime dans le meme clone. Ce nouveau gene presente une diversite clonale, code pour une chaine polypeptidique qui contient des domaines variable et constant de type immunoglobuline, porte des sites de N-glycosylation potentiels et constitue un candidat particulierement vraisemblable pour le gene codant pour la sousunite alpha du recepteur heterodimere pour l'antigene de ce clone cellule T

The nuclear RNA from a large variety of kappa-producing plasmacytomas was size fractionated and analyzed with a series of cloned probes representing sequences encoding variable (V), joining (J), and constant (C) regions and selected intervening sequences. All of the plasmacytomas produce a nuclear RNA component that contains V kappa and C kappa sequences as well as the intervening sequence between J kappa and C kappa, and that has a distinctive size depending on which of the four J kappa segments is expressed (i.e., is present in the secreted kappa chain). These RNAs are the precursors of kappa mRNAs, which are transcribed from productively rearranged C kappa genes. Half of the plasmacytomas examined produce, in addition to a kappa mRNA precursor, a discrete component of about 8.4 kilobases that contains C kappa and upstream flanking sequences but lacks the expressed V region sequence. The ability to produce this component is always associated with the persistence in the tumor genome of an unrearranged (germline) J kappa-C kappa region. In tumors rearranged at both kappa loci the nonproductive allele is either transcriptionally silent or, in a minority of cases, transcribed and processed into a "fragment" mRNA lacking V region sequences. These results reveal that allelic exclusion can be effected at several levels of gene expression. They also provide some insight into the relative contributions of the V and C gene elements to this expression.

A tissue-specific transcriptional enhancer element that is associated with the human immunoglobulin heavy-chain locus is defined. In a non-Hodgkin's lymphoma that contains a translocated c-myc gene this enhancer is retained on the 14q+ chromosome and occurs within sequences shown to activate previously cryptic promoters of the c-myc gene.

A cDNA clone encoding the alpha chain of the human T-cell antigen receptor was isolated by screening a library from the human T-cell line Jurkat with a mouse alpha-chain cDNA clone. This human alpha-chain clone, together with a human antigen receptor beta-chain cDNA clone, was used to determine the stage of T-cell development at which antigen receptor mRNAs first appear. Blot-hybridization analysis of mRNA isolated from a panel of human thymic tumor lines clearly demonstrated that beta-chain transcripts could be detected in all T-lineage cells. However, alpha-chain transcripts were only found in the most phenotypically mature lines, which express the antigen receptor-associated molecule T3. Furthermore, beta-chain transcripts were abundant in RNA prepared from purified T3-negative thymocytes, whereas alpha-chain transcripts were virtually absent. From these results we conclude that alpha-chain expression occurs later in thymic ontogeny than that of the beta chain and propose that it controls surface expression of the antigen receptor-T3 complex.

One day in the fall of 1970, I received an airmail letter from Renato Dulbecco who was traveling in Europe. At that time I was a postdoctoral fellow in his laboratory at the Salk Institute. The letter, written on stationery of the Hotel Hassler in Rome, said