Sipa1,previously calledSpa1,is transcriptionally induced in the murine lymphoid cells following mitogenic stimulation and encodes a protein with a domain related to Rap1 GTPase activating protein (Rap1GAP) at the N-terminus and to PEST sequences followed by a leucine zipper motif at the C-terminus. Herein mouse genomicSipa1,which consisted of 16 exons, was cloned. Gene linkage analysis using (BXD) recombinant inbred strains indicated thatSipa1was mapped to the most centromeric region of chromosome 19 syntenic with the long arm of human chromosome 11. HumanSIPA1cDNA exhibited a striking homology to that of mouse throughout the entire region, with the overall identity being 90% at the amino acid level. Human genomic clones, which hybridized with both mouse and humanSIPA1cDNA but not withRAP1GAPcDNA, were then isolated. Fluorescencein situhybridization (FISH) analysis using the human genomic clones indicated thatSIPA1was indeed mapped to chromosome 11q13, most likely to the 11q13.3 subregion. It was further indicated by double-color FISH thatSIPA1was located in the centromeric neighborhood ofCCND1/PRAD1,a presumedBCL1oncogene.