Genetically engineered mouse models (GEMMs) have become indispensable tools in cancer research, with their utility ranging from studies of the basic biology of tumorigenesis and progression to the creation of genetically accurate tumor models for the evaluation of novel therapeutic approaches. The objectives of the L.T. Christian III Transgenic/Knockout Mouse Shared Resource are to: 1) facilitate the development of new GEMMs (including transgenic, knockout, conditional knockout and knock-in mice) using both conventional and CRISPR-Cas9-based technologies, 2) provide a full range of support services to facilitate the use of these models, including breeding colony management, mouse genotyping, mouse line rederivation, embryo and sperm cryopreservation and cryorecovery, and 3) offer expert consultation on the development and use of GEMMs.