The times they are a-changin' for GEM and the facilities that maintain them. Core facilities are expanding beyond their original conception as producers of transgenic mice to encompass a wide range of services, including research animal maintenance. In this paper, the authors describe the logistics and administration of the newly dubbed Mouse Genetics Core Facility at the Memorial Sloan-Kettering Cancer Center as a blueprint for other institutions seeking to expand and update their own transgenic cores for research in the twenty-first century.