Avian carcinoma virus MH2 contains a transformation-specific sequence, mht, and shares the myc sequence with MC29, CMII, and OK10 viruses.

Avian carcinoma virus MH2 has been grouped together with MC29, CMII, and OK10, because all of these viruses share a transformation-specific sequence termed myc. A 5.2-kilobase (kb) DNA provirus of MH2 has been molecularly cloned. The complete genetic structure of MH2 is 5'-delta gag(1.9-kb)-mht(1.2-kb)-myc(1.3-kb)-delta env(?) and noncoding c-region (0.2-kb)-3'. delta gag, delta env, and c are genetic elements shared with nondefective retroviruses, whereas mht is a unique, possibly MH2 transformation-specific, sequence. Hybridizations with normal chicken DNA and cloned chicken c-myc DNA indicate that the mht sequence probably derives from a normal cellular gene that is distinct from the c-myc gene. The genetic structure of MH2 suggests that the delta gag and mht sequences function as a hybrid gene that encodes the p100 putative transforming protein. The myc sequence of MH2 appears to encode a second transforming function. Therefore, it seems that MH2 contains two genes with possible oncogenic function, whereas MC29, CMII, and OK10 each carries a single hybrid delta gag-myc transforming gene. It is remarkable that, despite these fundamental differences in their primary structures and mechanisms of gene expression, MH2 and MC29 have very similar oncogenic properties.