SHM generates high affinity antibodies in response to antigenic challenge; it does so by introducing point mutations into the antigen-binding regions of B cell antibody genes. Mutations at C-G base pairs during SHM are the work of the enzyme AID (activation-induced cytidine deaminase), which turns cytosine into uracil. A-T mutations have been harder to explain. Error-prone polymerases are thought be the culprits behind A-T mutation, but specific roles for these enzymes have been difficult to assign, as mice lacking individual polymerases have thus far shown no defects in SHM.

Pol has been the primary suspect charged with mutating A-T base pairs, as the pattern of errors made by pol in vitro is reminiscent of that seen in mutated Ig loci. In addition, humans lacking pol have fewer A-T mutations in their Ig genes than normal. Delbos et al. now solidify the evidence by showing that elimination of pol in mice decreases Ig gene mutations at A-T base pairs.

The few A-T mutations that occurred in the absence of pol—to the authors' surprise—bore the signature of another polymerase, pol, which does not normally meddle in SHM. The authors suggest that the mismatch repair protein complex MSH2–MSH6, recently shown to recruit pol to AID-induced U-G mismatches, may in pol's absence instead bind pol. The pol would then cause mutations at nearby A-T sites.