Several projects in this application require the generation of genetically modified mouse embryonic stem (ES) cell lines. Gene targeting experiments are technically challenging and time-consuming even under the best circumstances. This is particularly true for laboratories that do not perform these experiments on a regular basis. One major concern is ES cell cultures. Although ES cells might appear morphologically normal, they might lose their ability to contribute to germline transmission of a mutation due to sub-optimal culture conditions. It can take several months to identify ES cell clones that do not contribute to germline transmission. It is therefore desirable to conduct these experiments in a central facility with experienced personnel. This will ensure that these experiments are done in the most efficient manner with respect to costs, time and quality. The ES Core will provide expertise in the design of targeting vectors and will perform all ES cell manipulations. This serve will include the generation and amplification of targeted ES cell clones and the preparation of cells for blastocyst injection (which will be done in Core B, Director, Franco DeMayo). The principled investigators will supply the targeting construct and will be responsible for screening the ES cell DNA for correct targeting.