The proximal promoter region of the gene for the CXCL13/BLC chemokine has been studied by electrophoretic mobility shift assay and reporter gene analysis in order to detect new control elements, in particular, NF-_κB binding sites. Two new putative control elements have been identified. One of them contains a Rel/ NF-_κ B binding site and seems to participate in inducible gene expression. The other is an Sp1 factor binding site, essential for basal transcription. It is the first time that such a site is found in the promoter of a target gene of the alternative NF- B pathway. This finding indicates that genes under the control of the alternative NF-_κB pathway can be cooperatively regulated by Rel/ NF-_κ B and Sp1. Our results will add to the understanding of the signaling pathways that govern the expression of genes controlled by the alternative NF-_κB pathway.