The myc family of cellular oncogenes contains three well-defined members: c-myc, N-myc and L-myc. Additional structural and functional evidence now suggests that other myc-family oncogenes exist. The overall structure and organization of the c-, N-, and L-myc genes and transcripts are very similar. Each gene contains three exons: encoding a long 5' untranslated leader and a long 3' untranslated region. The proteins encoded by these myc genes share several stretches of significant homology. The conservation of sequences at the carboxyterminus of the L-myc protein suggests that it is also a DNA-binding, nuclear-associated protein. Each myc gene will cooperate with an activated Ha-ras oncogene to cause transformation of primary rat embryo fibroblasts. Characteristics of several new myc-family members are described.