Immunoglobulin genes are assembled during lymphoid development by a series of site-specific rearrangements that are tightly regulated to ensure that functional antibodies are generated in B (but not T) cells and that a unique receptor is present on each cell. Because a common V (D) J recombinase comprising RAG1 and RAG2 proteins is used for both B-and T-cell antigen receptor assembly, lineage-specific rearrangement must be modulated through differential access to sites of recombination. We show here that the C-terminus of the RAG2 protein, although dispensable for the basic recombination reaction and for Ig heavy chain D H to J H joining, is essential for efficient V H to DJ H rearrangement at the IgH locus. Thus, the RAG2 protein plays a dual role in V (D) J recombination, acting both in catalysis of the reaction and in governing access to particular loci.