Class switch recombination (CSR) is a region-specific DNA recombination reaction that replaces one immunoglobulin heavy-chain constant region (Ch) gene with another. This enables a single variable (V) region gene to be used in conjunction with different downstream Ch genes, each having a unique biological activity. The molecular mechanisms that mediate CSR have not been defined, but activation-induced cytidine deaminase (AID), a putative RNA-editing enzyme, is required for this reaction. Here we report that the Nijmegen breakage syndrome protein (Nbs1) and phosphorylated H2A histone family member X (γ-H2AX, also known as γ-H2afx), which facilitate DNA double-strand break (DSB) repair^,,, form nuclear foci at the Ch region in the G1 phase of the cell cycle in cells undergoing CSR, and that switching is impaired in H2AX^-/- mice. Localization of Nbs1 and γ-H2AX to the Igh locus during CSR is dependent on AID. In addition, AID is required for induction of switch region (Sµ)-specific DNA lesions that precede CSR. These results place AID function upstream of the DNA modifications that initiate CSR.