The recombinase activating genes, RAG-1 and RAG-2, are essential for rearrangement of antigen receptor gene segments and the precise expression of these genes during discrete stages of B lymphopoiesis is critical to this process. In order to identify elements controlling the transcription of the RAG locus, we are investigating RAG-1 and RAG-2 expression in murine pre-B cell lines. Our approach is based on using cells lines that do not express the RAG genes and variants of them that express one or both of the RAG genes. Southern blot analysis of such paired variants indicated that both cell lines contain intact RAG-1 and RAG2 genes ruling out gross changes in the gene structure as the reason for the varying expression. A closer examination of the loci in these cells is ongoing. Further experiments to determine the reason for the variant RAG expression include assays that identify transcription factors and utilize known signaling pathways. In companion studies, the 7 kb region of DNA between the murine RAG genes is being sequenced in order to identify potential regulatory regions.