The rap1/Krev-1 gene (RAP1A) encodes a p21-related protein that is able to suppress transformation by activated p21ras. The GTPase activating protein (GAP) gene for p21rap1A (RAP1GA1) has recently been assigned to chromosome 1p36.1-p35, a region of the genome that is frequently involved in cytogenetic deletions and rearrangements in several different tumors including melanoma, neuroblastoma, breast and lung carcinomas. GAP genes negatively regulate the activity of p21 proteins by catalyzing the conversion of the active GTP-bound forms to the inactive GDP-bound forms. The physiological function of p21rap1A-GAP makes it a strong candidate as a tumor suppressor gene that may have a role in the development of one or more of these malignancies. In the past year, we have refined the localization of RAP1GA1, and determined the complete intron/exon structure of the gene as a preliminary step TP examining the role of RAP1GA1 in tumorigenesis.