Pax-5 is identical to EBB-1/KLP and binds to the VpreB and lambda5 promoters as well as the KI and KII sites upstream of the Jkappa genes.

During B cell differentiation, the pre-B cell stage plays a significant role in immunoglobulin gene rearrangement in the context of the allelic exclusion and kappa chain gene rearrangement. We previously reported that the early B cell-specific binding protein (EBB)-1 transcription factor binds to the promoters of two pre-B cell-specific genes, VpreB and lambda5, and regulates their pre-B cell-specific expression. Here, we demonstrate that EBB-1 binds to the KI and KII sites in the upstream of Jkappa region, which are crucial for kappa chain gene rearrangement. Gene transfer and gel-shift assays demonstrate that EBB-1 is identical to Pax-5 and binds to promoters of VpreB and lambda5 as well as the KI and KII sites. Our results suggest that Pax-5 plays an important role in the coordinate regulation of several immunoglobulin gene family members that are crucial in B cell development.