REV1 acts as a deoxycytidyl transferase to incorporate a single dCMP opposite a damaged DNA residue. REV1 most efficiently incorporates dCMP opposite apurinic/apyrimidinic (AP, abasic) sites. REV1 enables DNA damage bypass without repair of damaged DNA bases, but its low fidelity results in a mutagenic effect (Nelson et al. 1996, Lin et al. 1999, Gibbs et al. 2000, Zhang et al. 2002).