Avian erythroblastosis virus (AEV) contains two distinct oncogenes, erbA and erbB . The erbB oncogene, which is homologous to a portion of the epidermal growth factor receptor, is related to the src family of oncogenes and efficiently transforms erythroblasts, whereas erbA potentiates the effects of erbB by blocking the differentiation of erythroblasts at an immature stage. This "potentiator" was sequenced; the amino acid sequence deduced from it was clearly different from the sequences of other known oncogene products and was related to carbonic anhydrases. These enzymes participate in the transport of carbon dioxide by erythrocytes, the precursors of which are main targets of avian erythroblastosis virus. A src-related oncogene such as erbB in synergy with an activated specific cell-derived gene such as erbA can profoundly affect early erythroid differentiation.