Activation-induced deaminase (AID) deaminates deoxycytidine residues in immunoglobulin genes, triggering antibody diversification. Here, by use of two-hybrid and coimmunoprecipitation assays, we identify CTNNBL1 (also known as NAP) as an AID-specific interactor. Mutants of AID that interfere with CTNNBL1 interaction yield severely diminished hypermutation and class switching. Targeted inactivation of CTNNBL1 in DT40 B cells also considerably diminishes IgV diversification. CTNNBL1 is a widely expressed nuclear protein that associates with the Prp19 complex of the spliceosome, interacting with its CDC5L component. The results, therefore, identify residues in AID involved in its in vivo targeting and suggest they might act through interaction with CTNNBL1, giving possible insight into the linkage between AID recruitment and target-gene transcription.