During lymphocyte development, V(D)J recombination is highly regulated in the contexts of lineage specificity, developmental-stage specificity, and allelic exclusion (4, 5). In addition, developing lymphocytes use strategies that require productive V(D)J rearrangements for continued developmental progression (4, 5). For example, Ig heavy chain (IgH) variable-region genes are assembled in progenitor (pro) B cells via an ordered process in which D H -to-J H rearrangement occurs on both alleles before V H -to-DJ H rearrangement. Productive V H -to-DJ H rearrangements produce cell-surface expression of IgH chains that signal expansion and differentiation of pro-B cells to the pre-B cell stage. This expression of IgH chains also signals the cessation of further V H -to-DJ H rearrangement via feedback regulation. Pro-B cells that first assemble nonproductive V H -to-DJ H rearrangements proceed to rearrange their second allele.Both RAG1 and RAG2 are required for V(D)J recombination, because RAG1-or RAG2-deficient mice exhibit a complete block in lymphocyte development at the progenitor stage (6, 7). In addition, RAG1 and RAG2 together are sufficient to initiate V(D)J recombination in vitro (8). Nearly all in vitro studies of RAG function have used the minimal regions of RAG1 (amino acids 384-1,008 of 1,040) and RAG2 (amino acids 1-383 of 527) required for activity, because full-length RAG1 and RAG2 are largely insoluble. However, the activities of these mutant ''core'' proteins differ from those of the full-length RAGs when assayed with extrachromosomal substrates in transfected cells. In this context, the mutant core RAG proteins support V(D)J recombination with reduced efficiency and with different levels and types of recombination products (9-15).Although not required for the biochemistry of V(D)J recombination, the noncore regions of RAG1 and RAG2 are conserved throughout evolution. For example, sequence conservation across the entire RAG2 protein from pufferfish to humans is Ϸ60%, with conservation of the noncore C-terminal region slightly higher than the core domain (16). Therefore, the noncore regions of RAG1 and RAG2 may serve important accessory and͞or regulatory functions in chromosomal V(D)J recombination. Consistent with this notion, studies of Abelson murine leukemia virus-transformed pre-B cells have suggested that the noncore regions of RAG1 are important for IgH locus D H -to-J H rearrangement (17), whereas the C terminus of RAG2 is more important for V H -to-DJ H rearrangement than for D H -to-J H rearrangement (18). The C terminus of RAG2 is predicted to fold into a plant homeodomain (PHD) (19,20), a motif found in a number of chromatin-associated proteins including some that have chromatin-modifying activities (21-24).To investigate potential in vivo function of the RAG2 noncore region in chromosomal V(D)J recombination and normal lymphocyte development, we have generated and characterized mice containing specific replacement of the full-length endogenous RAG2 gene with a gene encoding the mouse core...