Embryonic stem (ES) cells have the developmental capability to contribute to every cell type of the adult mouse when reintroduced into the blastocyst embryo (1,2). They can be easily manipulated in vitro to produce specific genetic alterations for the study of gene function and the biological basis of the diseased state. More recently, human ES (3–5) and embryonic germ (6) (EG) cells have been isolated; these cells have the potential to provide new therapeutic treatments in a broad range of diseases (7). The methods used for human ES isolation are similar to those used for mouse ES derivation (8). Although ES-like cells have been reported in rabbit (9), pig (10), cattle (11), rhesus monkey (12), and human (3–5), as yet, only a few strains of mice (predominantly, strain 129 and C57BL/6) (13) have successfully produced totipotent ES cells capable of colonizing the germline. There is an apparent genetic variation even among mouse strains to the relative ease with which ES cells can be isolated (14).