Hereditary multifocal renal cystadenocarcinoma and nodular dermatofibrosis (RCND) is a naturally occurring canine kidney cancer syndrome that was originally described in German Shepherd dogs. The disease is characterized by bilateral, multifocal tumors in the kidneys, uterine leiomyomas and nodules in the skin consisting of dense collagen fibers. We previously mapped RCND to canine chromosome 5 (CFA5) with a highly significant LOD score of 16.7 (theta=0.016). We have since narrowed the RCND interval following selection and RH mapping of canine genes from the 1.3 x canine genome sequence. These sequences also allowed for the isolation of gene-associated BACs and the characterization of new microsatellite markers. Ordering of newly defined markers and genes with regard to recombinants localizes RCND to a small chromosomal region that overlaps the human Birt-Hogg-Dubé locus, suggesting the same gene may be responsible for both the dog and the phenotypically similar human disease. We herein describe a disease-associated mutation in exon 7 of canine BHD that leads to the mutation of a highly conserved amino acid of the encoded protein. The absence of recombinants between the disease locus and the mutation in US and Norwegian dogs separated by several generations is consistent with this mutation being the disease-causing mutation. Strong evidence is provided that the RCND mutation may have a homozygous lethal effect (P<0.01).