The developmental progression from pro-B to pre-B cells is controlled by pre-B cell receptor (pre-BCR) signaling which depends on BLNK (SLP-65) for coupling the Syk kinase to its downstream effector pathways. Here we identified BLNK as a direct target of the transcription factor Pax5 (BSAP). Restoration of BLNK expression in Igμ transgenic Pax5^−/− pro-B cells resulted in constitutive pre-BCR signaling and increased cell proliferation without inducing progression to the pre-B cell stage. Igμ⁺ Pax5^−/− pro-B cells expressing a BLNK-estrogen receptor fusion protein initiated signaling immediately upon hormone addition, which facilitated analysis of pre-BCR-induced gene expression changes. The pre-BCR was shown to execute its checkpoint function by regulating genes involved in cell proliferation, intracellular signaling, growth factor responsiveness, and V(D)J recombination.