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Structural and functional homologies have been found among proteins encoded by several retroviral oncogenes, demonstrating the existence of families of these genes¹. Because the retroviral oncogenes have cellular homologues^1,2, the existence of similar families among these ‘cellular oncogenes’ is also implied (for a review, see ref. 2). Cellular genes belonging to these families have been found in such evolutionarily distant species as humans^3,4, fruit flies^5,6, nematodes⁵ and brewer's yeast (E. Scolnick and S. Reed, personal communications), consistent with the hypothesis that these genes have evolved from a small number of ancestral sequences². We extend these observations by showing here that the proteins encoded by the oncogenes myc, myb and adenovirus E1a are structurally related. Our findings suggest that oncogenes of RNA and DNA tumour viruses may in at least some instances share …