This project will provide the capability to maintain pedigreed breeding colonies of feral mice and certain strains of transgenic mice. The mice will be used to test the effect of various biological [hormones, mouse mammary tumor virus (MMTV), and recombinant retroviral shuttle vectors] and chemical carcinogens on mammary gland development and tumorigenesis and to provide a source of tumor tissue to define the organization and to test for the expression of tumor-associated genes and retroviral genes.