Upon activation by antigen, B cells undergo antibody class (isotype) switching, changing from expression of IgM to expression of IgG, IgA or IgE, while maintaining specificity for the same antigen. Since the isotype determines the effector function of the antibody, class switching allows the humoral immune response to adaptively respond to different infectious organisms. Class switching occurs by a DMA recombination event between switch (S) region sequences located upstream of each heavy chain constant (CH) region gene. This process has mechanistic similarities to somatic hypermutation of Ig variable region genes. It has recently become clear that activation-induced cytidine deaminase (AID) initiates class switch recombination (CSR) by deamination of dC residues within S regions, creating dU residues. It is thought that single strand (ss) DNA nicks are then created by the base excision repair pathway, which converts dU residues to abasic sites, which are then nicked by AP endonuclease. We hypothesize that mismatch repair (MMR) enzymes recognize U:G mismatches created by AID, recruit Exonuclease 1 to the ss breaks created by base excision repair which excises a ss patch resulting in the conversion of these ss breaks to the double-strand breaks (DSBs) that are required for CSR. We also hypothesize that other mismatches and loops formed at S regions due to collapse of R-loops out-of-register will also recruit MMR proteins to S regions during CSR. There are three specific aims which have the goal of providing evidence for these hypotheses. Aim 1: To determine the roles of the mismatch proteins in CSR. Aim 2: To determine the role of Su tandem repeats and the function of their interaction with MMR proteins. Aim 3: To determine the role of uracil-DNA-glycosylase (UNG) in CSR. In Aim 4, we propose to address the hypothesis that Mlh1 interacts with other mouse MutS homologs during CSR due to our finding that Mlh1 has functions in CSR that are independent of Msh2. Specifically, we propose to investigate whether Msh5 has a role in CSR.