A lesion bypass model. The large oval represents replication proteins. Numbers 1–4 indicate four different lesion bypass mechanisms. The two-polymerase two-step hypothesis for lesion bypass is depicted as mechanism 4, and the Pol ζ dual-function hypothesis is illustrated in mechanisms 3. Squares in mechanisms 1 and 2, Polη and Polκ, respectively; green oval in mechanisms 3 and 4, Polζ; rectangle and diamond in mechanism 3, putative accessory factors of Polζ.