MUTYH initiates the excision of the misincorporated adenine opposite 8-oxo-dG. The resulting 1-nt gap is then filled by Pol λ in an error-free (C) or error-prone (A) manner. Completion of the canonical short-patch (SP) pathway is futile as DNA ligase III cannot seal an 8-oxo-dG:dC base pair at the primer terminus. Thus, Pol λ promotes the long-patch (LP) extension past a correct base pair to complete error-free BER. Conversely, Pol λ is capable of stalling ligation (SP) or extension (LP) of an 8-oxo-dG:dA mispair, thus promoting its excision by proofreading. The resulting intermediate can then proceed through the error-free pathway. 