An update on the role of translesion synthesis DNA polymerases in Ig hypermutation.

Several years have passed since the discovery of activation-induced cytosine deaminase (AID), the molecule responsible for triggering hypermutation of Ig genes. We now know that AID deaminates cytosines in the DNA encoding the variable portion of the Ig receptor, although an additional role in deaminating a regulatory mRNA transcript has not been ruled out. A major question that remains unanswered is how AID, a cytosine deaminase, causes mutations at both G:C and A:T base pairs. Mounting evidence suggests the involvement of a group of error-prone DNA polymerases known to bypass DNA lesions: the translesion synthesis (TLS) DNA polymerases. In this Review, we discuss the evidence for a role of TLS DNA polymerases in Ig hypermutation and argue that a major remaining challenge in our understanding of this mechanism is the recruitment of TLS DNA polymerases to the Ig locus following AID-mediated cytosine deamination.