The v-maf oncogene of the avian musculoaponeurotic fibrosarcoma virus, AS42, encodes a nuclear protein which contains a characteristic b-Zip domain. By screening a chicken embryo fibroblast (CEF) cDNA library under moderately stringent hybridization conditions, we picked up a series of cDNA clones for a novel maf-related gene which we named mafK. We also identified another maf-related gene named mafF by screening a chicken genomic library using a mafK probe. Structural analyses suggested that the mafK and mafF genes consist of three exons. The exon-intron structures of the two genes resemble each other, but differ from that of the chicken c-maf gene. As compared to the c-Maf protein, the proteins encoded by the mafK and the mafF genes are rather small in size and lack the regions corresponding to the amino terminal acidic domain present in the c-Maf protein. On the other hand, the structures of the b-Zip domain are well conserved among these Maf-related proteins. When overexpressed by using an avian retroviral vector, the two maf-related genes did not induce morphological transformation of CEF cells but induced colony formation in soft agar with very low efficiencies. With a specific antibody, the MafK protein was detected predominantly in the nuclei of the cells infected with the virus which carries the mafK gene. Tissue distributions of these three maf-family genes are different from one another, probably reflecting their different functions in vivo.