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We have generated transgenic mouse lines that carry one of three different constructs in which the murine N-myc gene is expressed under the control of the immunoglobulin heavy chain transcriptional enhancer element (E mu-N-myc genes). High-level expression of the E mu-N-myc transgenes occurred in lymphoid tissues; correspondingly, many of these E mu-N-myc lines reproducibly developed pre-B- and B-lymphoid malignancies. The E mu-N-myc transgene also appeared to participate in the generation of a T cell malignancy that developed in one E mu-N-myc mouse. These tumors and cell lines adapted from them expressed exceptionally high levels of the E mu-N-myc transgene; the levels were comparable to those observed in human neuroblastomas with highly amplified N-myc genes. In contrast, all of the E mu-N-myc cell lines had exceptionally low or undetectable levels of the c-myc RNA sequences, consistent with the possibility that high-level N-myc expression can participate in the negative 'cross-regulation' of c-myc gene expression. Our findings demonstrate that deregulated expression of the N-myc gene has potent oncogenic potential within the B-lymphoid lineage despite the fact that the N-myc gene has never been implicated in naturally occurring B-lymphoid malignancies. Our results also are discussed in the context of differential myc gene activity in normal and transformed cells.

The myc-family of cellular oncogenes is a dispersed multi-gene family that includes the c-, N- and L-myc genes (For review see Alt et al., 1986). The human and murine c-, N, and L-myc genes encode related but distinct nuclear proteins and have a similar overall organization (Kohl et al., 1986; DePinho et al., 1986, Stanton et al., 1986; DePinho et al., 1987; Legouy et al., 1987; Kay et al., 1988); all have three exons with the first encoding a potentially untranslated leader sequence. All three genes also cooperate similarly with an activated Ha-Ras oncogene to transform primary rat embryo fibroblasts (REFs) (Yancopoulos et al., 1985; DePinho et al., 1987). Despite striking similarities of regions of the myc proteins and the in vitro transforming activities, the three genes are conserved as distinct sequences throughout vertebrate species suggesting unique functional roles. This possibility is supported by findings that the genes are differentially expressed in a stage-and tissue-specific manner during human and murine differentiation (Zimmerman et al., 1986). In addition, deregulated c-myc expression has been causally implicated in the genesis of a wide variety of different tumor types and occurs by a variety of different mechanisms; deregulation of the N- and L-myc genes has been clearly implicated only in a few naturally occurring tumors (eg. human neuroblastomas and small cell lung carcinomas) and only by the mechanism of gene amplification (reviewed by Alt et al., 1986).

Sequences of genes, and gene products such as proteins, provide a detailed and largely self-consistent context for discussion of molecular phylogenies and taxonomic relationships. Analysis at the DNA level, an alternative and often more rapid approach for protein sequence analysis, requires identification of codogenic regions. We describe how protein sequencing, sequencing of peptide mixtures, and amino acid analysis can be used to complement DNA sequencing. We demonstrate the usefulness of protein structural studies for the assignment of initiation codons to protein genes, of exons and introns to DNA sequences and for the identification of sequence internal residues such as active site residues and post-translational modifications. In addition, we show that partial protein sequencing is very useful for proving the existence of a gene product if neither specific antisera nor any functional assay exists. For proteins belonging to a known family, sequence comparison by indirect methods such as the comparison of amino acid compositions, may permit an accurate, inexpensive, and rapid description of phylogenies and taxonomic relationships. Finally, we discuss the characteristics of naturally occurring DNA sequences and the codon distribution of known reading frames in prokaryotic and eukaryotic genes using a nucleotide sequence data base of 600,000 base pairs.