Mast cell tumors (MCT) are the most common skin tumors in dogs. Survival rates are dependent upon the tumor grade. While dogs with grade 1 tumors often experience long-term survival, those with grade 3 tumors usual die within six months. This cancer has a particularly high occurrence in certain breeds, which suggests a significant genetic component. This study will use a new approach to identify genes that contribute to an increased risk of developing mast cell cancer in golden retrievers and related breeds. Identifying these genes will help scientists rapidly develop genetic tests that would indicated a predisposition to this cancer and would help develop therapeutic strategies based on tumor grade.