The Gene Targeting Shared Resource assists both first time users and experienced researchers to alter gene function through homologous recombination in embryonic stem (ES) cells followed by re-introduction of these cells into the mouse germline through chimera formation. Services include maintenance of ES cells competent to re-colonize the germline with high efficiency, advice upon construct design, microinjection of ES cells into blastocysts, and chimera analysis. To support these activities, stocks of various ES cell lines are maintained along with core mouse colonies sufficient to generate enough blastocysts for injection, as well as pseudopregnant recipient mice for the blastocyst transfers and vasectomized males to mate with these females. Dr. Jeffrey Pollard became faculty supervisor when Dr. Ronald DePinho left Einstein in 1998. Dr. Pollard has had over twenty years of experience in mouse biology and genetics and has worked extensively on the phenotypic characterization of null mutant mice. Dr. Winfried Edelmann, deputy faculty supervisor, has made over twenty induced mutant strains (knock-outs) of mice including the construction of many mice with point mutations in mismatch repair genes. The facility has been exceptionally productive having generated more than 100 induced mutations in mice since its inception. There is a very high success rate in obtaining germline chimeras through the expert skills of Mr. Harry Hou, who has been the cell-injector since the inception of the facility. Indeed, many investigators from other major institutions around the U.S. have collaborated with the AECCC facility to establish important induced mutant models for their genes. The facility is intimately involved in the implementation of new technologies and the training of new investigators in the technical and theoretical aspects of the procedures. Over the last granting period several innovations were introduced into the facility including the use of gridded BAC 129 strain genomic libraries for the rapid isolation of genomic fragments, the use of cre-lox systems to allow temporal and spatial ablation of genes, and bacterial recombination methods to rapidly modify targeting constructs. All these methodologies permit the more rapid generation of induced mutations in mice.