In susceptible strains of mice, leukemia is caused by the somatic integration of murine leukemia retroviruses into the host genome. Integration sites that are common to several tumors are likely to affect genes that are important in oncogenesis. Here we present the analysis of a common site of retroviral integration on mouse chromosome 15, which includes the genomic structure of three genes near the integration site. One of the genes misexpressed at the insertion site has recently been characterized as a B-cell receptor, Tnfrsf13c (formerly Baffr), indicating that this approach is useful in defining genes that function in lymphocyte development and tumor progression. Current genome databases provide powerful resources for the rapid identification of genes at common proviral insertion sites. The characterization of these genes in tumor samples will allow a function to be assigned to many novel loci identified by the genome sequencing projects.