Chimeric mice often are created through the genetic manipulation of the mou
se embryo in the process of developing animal models of disease. These mice
have variable percentages of their somatic and germ cells derived from the
donor embryonic stem cells and host blastocysts. In the development of mou
se models deficient in the breast cancer susceptibility gene 2 (Brca2) or t
he 70-kd heat shock protein (Hsp70-2), 3-4-week-old chimeras developed sing
le or multiple masses composed of both well-differentiated and poorly diffe
rentiated tissues derived from all three germ layers. These cases of extrag
onadal teratocarcinoma, a rarely reported tumor, may be related to the gene
tic predisposition of the 129/O1a mouse strain used to generate the embryon
ic stem cells.