The genome of avian erythroblastosis virus contains two independently expressed genetic loci (v-erbA and v-erbB) whose activities are probably responsible for oncogenesis by the virus. Both loci are closely related to nucleotide sequences found in the DNA and RNA of chickens and other vertebrates. We have isolated and characterized chicken DNA homologous to v-erbA and v-erbB. The two viral genes are represented by separate domains within chicken DNA (c-erbA and c-erbB), which are separated by a minimum of 12 kilobases (kb) of DNA and may not be linked at all. The nucleotide sequences shared by the viral and cellular erb loci are colinear, but the cellular loci are interrupted by multiple intervening sequences of various lengths. Polyribosomes prepared from normal chicken embryos contain two polyadenylated RNAs transcribed from c-erbA and two transcribed from c-erbB. The evident coding regions of these RNAs represent an unusually small fraction of the lengths of the RNAs, as if the 3' untranslated domains of the RNAs might be exceptionally large (3-11 kb). These findings indicate that the c-erb loci are normal vertebrate genes rather than genes of cryptic endogenous retroviruses, and that they may have a role in the metabolism of normal cells. It appears that the viral erb genes, like most other retrovirus oncogenes, have been copied from cellular genes. In the viral genome, the two genes are devoid of introns, but they remain independently expressed loci, and they remain colinear with the coding domains of their cellular progenitors.