Gene targeting in embryonic stem (ES) cells is being used to inactivate (knock-out) genes and use the mutated ES cells to generate mice with a mutation at the targeted locus. We have targeted the pro-opioelanocortin (POMC) gene in embryonic stem cells in order to determine the influence of POMC during development and post-natally. The POMC-targeted ES cells are now being used to generate chimeric animals that may carry the gene defect in the germline. Similarly, the cystatin C gene has been cloned and gene targeting constructs have been made in order to inactivate the cystatin C gene in ES cells. Cystatin-C mice will be made and the mutated human cystatin C gene from a patient with hereditary cerebral angiopathy will be introduced into one of the constructs in order to make a mouse model for hereditary stroke.