Recombination activating gene 1 (RAG1) and RAG2 are critical enzymes for initiating variable-diversity-joining [V(D)J] segment recombination, an essential process for antigen receptor expression and lymphocyte development. The BCL11A transcription factor is required for B cell and plasmacytoid dendritic cell (pDC) development, but its molecular function(s) in early B cell fate specification and commitment is unknown. We show here that the major B cell isoform, BCL11A-XL, binds directly to the RAG1 promoter as well as directly to regulatory regions of transcription factors previously implicated in both B cell and pDC development to activate RAG1 and RAG2 gene transcription in pro-and pre-B cells. We employed BCL11A overexpression with recombination substrates to demonstrate direct consequences of BCL11A/RAG modulation on V(D)J recombination. We conclude that BCL11A is a critical component of a transcriptional network that regulates B cell fate by controlling V(D)J recombination.KEYWORDS B cell development, Bcl11a, immunology, RAG, V(D)J R ecombination activating gene (RAG) endonuclease catalyzes the cleavage phase of V(D)J recombination (1-4). RAG is encoded by two adjacent genes, RAG1 and RAG2, whose products must interact to form an active endonuclease (5). Both RAG1 and RAG2 are essential for subsequent B and T lymphocyte development (4, 6). In cell-free systems, RAG1 and RAG2 are sufficient to cleave recombination substrates (7,8), but in vivo a number of additional factors are required for chromatin accessibility of V(D)J segments and for appropriate completion of the V(D)J joints (1-3).RAG expression occurs at two distinct points during B cell development. The first phase results in the assembly of the immunoglobulin heavy chain (IgH) in pro-B cells, whereas the second catalyzes Ig light chain (L) assembly in pre-B cells. RAG expression is tightly regulated at both the posttranscriptional and transcriptional levels (2). In addition to the individual promoters for RAG1 and RAG2, the RAG locus contains at least