This year, we provided the first comprehensive comparison of the efficiency of MMR of leading and lagging strand replication errors. We demonstrated that exonuclease 1 preferentially repairs mismatches generated by DNA polymerase alpha. We provided strong evidence that ribonucleotides are signals for mismatch repair of leading strand replication errors. Finally, we collaborated with Winfried Edelmann on a study indicating that mouse Exo1 encodes both structural and catalytic functions that play distinct roles in several essential biological processes.