We have isolated chicken genomic and cDNA clones representing the jun oncogene of avian sarcoma virus 17 (ASV17). The genomic clone lacks intron sequences within its protein coding domain, contains a CAAT box, seven SP-1 consensus sequences and TATA box-like elements upstream and two poly(A) addition signals downstream of the coding domain. The cellular jun protein is 310 amino acids in length. Cellular and viral jun proteins differ by three nonconservative amino acid substitutions of which two are located in the DNA-binding domain, by a 27-amino-acid deletion in the amino terminal third of the viral jun protein, by eleven cell-coded amino acids that link the cellular jun coding domain to the viral gag domain and by the partial gag sequences constituting the amino terminal of the viral gag-jun fusion protein. The availability of a cellular jun cDNA now allows the construction of reciprocal recombinants between the viral and the cellular gene which will define the structural features required for the oncogenicity of v-jun.