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PROGRESS in somatic cell genetics now allows more rapid and precise localisation of genes within the human genome. A large number of mouse × human hybrid cell lines are available, permitting the investigator to choose, for phenotype assay, lines with a particular reduced human chromosome complement. An increasingly large number of hybrid lines carrying rearranged chromosomes are also becoming available for regional localisation studies. We have used such hybrid cell lines to assign a gene coding for galactokinase (EC 2.7.1.6) to human chromosome 17 and to further localise the gene to band 21–22 on the long arm of the chromosome. Results from this study have provided new information Regarding the functioning of the genes coding for the Leloir pathway enzymes in man1, and have revealed a relatively close and potentially useful linkage between the genes for galactokinase and for thymidine kinase (EC 2.7.1.21).

The homoeo box is a 180-base pair (bp) DNA sequence conserved in Drosophila homoeotic genes, which regulate early development1,2. These DNA sequences are present in open reading frames and have been identified in specific gene transcripts in Drosophila and Xenopus embryos3–5; they possess structural features in common with genes encoding some DNA-binding proteins6,7. Homologous homoeo box sequences have been detected in species ranging from insects and annelids to vertebrates2,5,8. The high degree of sequence conservation (70–90%) among different species suggests a strong evolutionary relationship and implies a common role in embryonic development. To test this hypothesis, one approach we have used is to examine the patterns of genetic organization of homoeo box sequences in mouse and human for any similarities; the second approach is to localize the chromosomal map positions of homoeo box sequences in the two species. A similar genomic organization and chromosomal distribution of homoeo box sequences would argue for a conserved function and might shed light on their mechanism of action. Here, we describe experiments which show that two homoeo box loci map, respectively, to evolutionarily related regions on mouse chromosome 11 and human chromosome 17.

The three active α-globin genes of the mouse are physically linked at a locus little more than 25 kilobases long. These genes form a part of an unexpectedly large and dispersed multigene family, including at least two pseudogenes located on two different chromosomes. This finding suggests that related gene sequences can be dispersed from their primary loci and that their dispersion may be a critical factor in the evolution of higher organisms.

The human genes for thymidine kinase, galactokinase and procollagen type I, located on chromosome 17, have been transferred to cultured mouse cells. In unstable cell lines the transferred genetic material is shown to exist independently, whereas in stable cell lines the transferred material has become associated with chromosomes of the recipient cell. The size of the transferred genetic material exceeds 1% of the human haploid genome in some of the transformed cell lines. In addition, the data indicate a gene order of: centromere–galactokinase–(thymidine kinase, procollagen type I), on the long arm of human chromosome 17.

Techniques for the study of somatic cell genetics, and particularly those involving the expression of enzyme markers in hybrid cells, have already made possible a large number of gene-chromosome assignments. Genetic and family studies, as well as cellular studies on recombination and gene transfer, promise more and quicker results in the future.

The creation of a functional antibody gene requires the precise recombination of gene segments initially separated on the chromosome. Frequently errors occur in the process, resulting in the formation of a non-functional gene. The non-functional genes can be generated by incomplete rearrangements, frameshifts, or the use of pseudo V or J joining segments. It is likely that these aberrant rearrangements arise by the same mechanism as is used in generating functional genes, a process which we have suggested may involve unequal sister chromatid exchange. Aberrant rearrangements of immunoglobulin genes occur in normal lymphocytes and play a major part in allelic exclusion. However, it has recently been suggested that aberrant rearrangements involving immunoglobulin and non-immunoglobulin genes may be involved in tumorigenesis. This suggestion has been stimulated by the frequent occurrence of translocations involving chromosomes known to carry immunoglobulin genes in B-cell malignancies. The rearrangement of non-immunoglobulin DNA to the heavy-chain locus has recently been reported. Some aberrant rearrangements of the kappa locus appear to be due to rearrangements to sites that do not include the conventional sequence for V gene segment joining. Here we describe an aberrant kappa rearrangement that has led to the joining of DNA from chromosomes 15, 6 and 12, and so appears to be the result of chromosomal translocations or transpositions. As 15/6 or 15/12 translocations have frequently been found in mouse plasmacytomas (as have analogous translocations in human lymphocyte tumours) this aberrant kappa rearrangement may be unique to the plasmacytoma from which it was isolated.

Mammalian gene mapping techniques are now sufficiently advanced to contribute significantly to prenatal diagnosis and to human molecular genetics. Restriction fragment mapping can be used to place polymorphic genetic markers at random sites within the genome, and these sites used to assign genes responsible for disease conditions to a chromosomal region. Somatic cell genetic techniques can then be applied to saturate that region with additional restriction fragment markers, some of which will be closely linked to the disease gene. Closely linked restriction fragment markers, especially flanking pairs of markers, can act as predictors for the transmission of defective genes to offspring. A series of tightly linked flanking restriction markers might in addition contribute to the eventual isolation and cloning of the disease gene itself.

The striking number of human and murine immunodeficiency disorders which map to the X chromosome (Table 1) suggests that genes localized on this chromosome must have important roles in lymphocyte development. At least seven distinct disorders in the human and two in the mouse disrupt lymphocyte maturation, particularly that of B cells, at characteristic stages1,2. As functional genes mapping to the X chromosome in one mammal are found on the X chromosome in all other mammals3, the same genes regulating lymphocyte development are expected to be found on the X chromosome in mouse and man. Investigations into the possible mechanisms of these X-linked disorders have been hampered by the lack of molecular probes for the genes or gene products affected; because of this, and the possibility of correlating one or more of the several hundred B- or T-cell-specific genes4 with a specific mutation, we surveyed 15 different B- and T-cell-specific cDNA clones for localization to the X chromosome. We report here the characterization of one of these murine cDNA clones, which hybridizes with a large, X-linked gene family, designated XLR (X-linked, lymphocyte-regulated). We show that the XLR gene family is closely linked to the X-linked immunodeficiency described in the CBA/N mouse strain (xid)5, by restriction fragment length polymorphism (RFLP) analysis of DNA from mice congeneic for xid. This finding, together with data on the expression of the XLR locus in B cells (see accompanying paper6), indicates that this gene family either includes the locus defined by the xid mutation or is adjacent to it in a gene complex which may be important in lymphocyte differentiation.

A common feature of Drosophila homoeo box genes appears to be their spatially restricted expression patterns during morphogenesis. Using Northern blot analysis and in situ hybridization to mouse tissue sections, the spatially restricted expression of a newly identified mouse homoeo box locus, Hox-3, within the central nervous system of newborn and adult mice has been demonstrated.

Xeroderma pigmentosum (XP) is an autosomal recessive human disease in which affected individuals are prone to develop skin cancers after exposure to sunlight. Cells from XP patients are defective in DNA repair activity and are sensitive to UV light. Most XP individuals have a defect in the excision repair pathway for UV damage. Genetic studies have indicated that excision-defective cells fall into seven complementation groups (A-G). An eighth group of XP is known to be defective in post-replication repair. DNA repair enzymes are ubiquitous and can function across species boundaries. Primary mouse embryo fibroblasts have normal levels of DNA repair functions. We report here that somatic cell hybrids between primary mouse cells and SV40-transformed XP group A cells can express wild-type levels of DNA repair function. These hybrid cells segregate murine chromosomes in culture. The proportion of cells in a given hybrid cell line which can perform unscheduled DNA synthesis (UDS) correlated well with the percentage of the population retaining murine chromosome 4. This study presents the first example of a direct quantitative comparison of specific gene activity and chromosomal content on a cellular basis.

ANTICELLULAR sera able to block specific cell responses are important in the study of surface receptors. Particularly useful are antibodies against the human cellular antigens coded for by a specific chromosome, which can be prepared by immunising mice with mouse–human somatic cell hybrids containing defined human chromosomes1. We have applied this method to the study of the mechanism by which the presence of human chromosome 21 renders such hybrids sensitive to human interferon2. Interaction of an interferon with sensitive cells induces intracellular biochemical changes, including the formation of an inhibitor of protein synthesis, making the cell unable to support virus replication3,4. We have obtained antibodies to hybrid cells containing human chromosome 21, and these inhibit the response of live human cells to interferon. Our experiments indicate that chromosome 21 codes for a cell-surface component required as a specific receptor for human interferon.

ALTHOUGH in vitro investigations of the primary1 and secondary2,3 phases of the immune response have been reported, few investigators have used blood as a source of immunologically competent cells4–6. The use of blood instead of lymphoid organs as a source of cells enables the investigator to obtain multiple samples of a homogeneous cell population. This communication reports a preliminary approach to an immunological investigative system consisting of sensitized rabbit blood leucocytes and T-2 bacteriophage, a particulate antigen that can be assayed by a procedure which is relatively simple, sensitive and quantitative.

THE biological and clinical implications of variations of human chromosome lengths are poorly understood.

Genetic Analysis with Man–Mouse Somatic Cell Hybrids: Linkage between Human Lactate Dehydrogenase A and B and Peptidase B