The primary role of the Transgenic Mouse Core Facility (TMF) is to facilitate the use of mouse molecular genetics at MSKCC for in vivo studies on gene function in a variety of fields basic to cancer research, including immunobiology, development, cell growth and proliferation, cellular differentiation, cell-cell interactions, and malignant transformation. Currently, the TMF offers eleven technical services: (1) production of transgenic mice by DNA microinjection into fertilized eggs; (2) genotyping of founder mice by Southern blot hybridization; (3) production of chimeric mice by embryonic stem (ES) cell injection into blastocysts; (4) identification of germ line chimeras; (5) cryopreservation of embryos and (6) cryopreservation of sperm for the long term storage of transgenic, gene-targeted, and other mutant strains of mice; (7) mouse strain rederivation/recovery by embryo transfer and in vitro fertilization; (8) specialized mouse husbandry for the maintenance of transgenic, gene-targeted, and mutant strains of mice; (9) special embryological and surgical procedures; (10) provision of special mouse strains; and (11) consultation and training in the design of transgenes and targeting vectors, ES cell culture, and mouse husbandry and genetics.