The myc gene family proteins and their role in transformation and differentiation.

Several genes within the mammalian genome share homology with the transforming gene, v-myc, of the avian myelocytic leukemia viruses. Of these, c-myc, Nmyc and Lmyc have been shown to possess transforming activity in vitro and each is found to be aberrantly expressed in a variety of tumours. In normal tissues, expression of Nmyc and Lmyc is restricted to embryonic development and a few adult tissues whereas c-myc and Bmyc are very widely expressed. The proteins encoded by the various myc genes share substantial sequence homology. c-, N and Lmyc proteins are all nuclear phosphoproteins that possess very short biological half lives and bind DNA. All three possess identifiable motifs present in a number of other nuclear proteins involved in transcriptional regulation, and the possible role of myc proteins in gene regulation is discussed. Some experiments, however, favour a different role for c-myc, in DNA replication. Whatever their precise molecular functions may be, it is clear that myc genes play an essential role in the regulation of cellular proliferation.