The mutation spectra of (A) rev3, (B) rev1, and (C) rev1-AA strains expressing APOBEC3G during temperature shift. All three genotypes were devoid of CC to CG transversions. Deletion of either REV3 (A) or REV1 (B) resulted in a prominent CC to CT hotspot at position 718. In contrast, rev1-AA cells (C) exhibited frequent CC to CT transitions at position 857. In addition, CC to CT transitions were observed at position 530, whereas in WT cells this was a strong CC to CG hotspot. These data suggest that insertion of C opposite abasic sites was entirely attributable to the deoxycytidyltransferase activity of Rev1.