The cellular homolog of the onc sequences in the avian retrovirus reticuloendotheliosis virus strain T (v-rel) was studied by molecular cloning and nucleic acid hybridization. In contrast to v-rel sequences, which are 1.4 kilobase pairs long, the cellular homolog, c-rel, from line 15B chickens is at least 25 kilobase pairs long, with multiple apparent introns. A 4.0-kilobase polyadenylic acid-containing RNA transcript is the primary species of c-rel RNA present in uninfected chicken cells. Sequences at the 3' end of this c-rel RNA are not present in v-rel.