MUTAGENESIS induced by DNA damage in Sacchalamyces cerevisiae requires the products of the REV1, REV3 and REV7 genes(1), The Rev3 and Rev7 proteins are subunits of DNA polymerase-zeta(2) (Pol-zeta), an enzyme whose sole function appears to be translesion synthesis(3). Rev1 protein has weak homology with UmuC protein(4), which facilitates translesion synthesis in Escherichia coli by an unknown mechanism. We show here that Rev1 protein has a deoxycytidyl transferase activity which transfers a dCMP residue from dCTP to the 3’ end of a DNA primer in a template-dependent reaction. Efficient transfer occurred opposite a template abasic site, but similar to 20% transfer also occurred opposite a template guanine and similar to 10% opposite adenine or uracil; less than or equal to 1% was seen opposite thymine or cytosine. Insertion of cytosine opposite an abasic site produced a terminus that was extended efficiently by Pol-zeta, but not by yeast Poi-alpha.