The REV1 protein is a member of the growing family of translesion DNA polymerases. A cDNA of the human REV1 gene that we had originally isolated encoded 1250 amino acids residues, which was one amino acid shorter than previously reported ones. The shorter form of REV1 was named REV1S. All individuals examined expressed equivalent amounts of REV1S and REV1 mRNA, suggesting that the REV1S mRNA is a splicing variant. We show that the REV1S protein also possesses deoxycytidyl transferase activity that inserts a dCMP opposite a DNA template apurinic/apyrimidinic site. Deletion and point mutation analysis of the REV1S protein revealed that the domain required for deoxycytidyl transferase and DNA binding activities of the REV1S protein are located in a conserved domain of translesion DNA polymerases. This result indicates that the structure of the catalytic site of the deoxycytidyl transferase closely resembles that of the translesion DNA polymerases. Therefore, the molecular mechanism of the dCMP transfer reaction of the REV1S protein and maybe also the REV1 protein might be the same as that of the dNTP transfer reaction of the translesion DNA polymerases.