What is meant by “a highly chimeric mouse?
A chimera is defined by Wikipedia as an animal that has two or more different populations of genetically distinct cells that originated in different zygotes; if the different cells emerged from the same zygote, it is called a mosaic (which happens occasionally in transgenic founders but not in ES cell derived chimeras). ES cell chimeras are comprised of cell populations and tissues arising from both the host blastocyst strain and the ES cell clone introduced into the E3.5 blastocyst stage embryo. While ES cell contribution to organs and tissues is not immediately evident and must be determined experimentally, ES cell contribution to pigmented tissues such as eyes, skin and hair can be visualized in neonatal pups. An indication of ES cell contribution to coat color can be visualized at about postnatal day 10 when the hair begins to come in. For HM1 (129P2/Ola) derived pups, ES cell contribution is notable as early as postnatal day 3 as marbled color skin patterns emerge. For some C57BL/
