  For the purposes of this entry, however, the term “chimera” will be restricted to chimeras formed by the introduction of human pluripotent stem cells (hPSCs) or their more specialized derivatives into non-human animals.  Pluripotent cells are capable of turning into any kind of tissue, and stem cells are capable of renewing themselves indefinitely. So hPSCs theoretically provide an indefinitely renewable source of any kind of human tissue, thus offering tremendous potential for basic research, drug development, and regenerative medicine (NIH 2001, 5–21). The first sources of hPSCs were human embryos, and are thus referred to as human embryonic stem cells (hESCs). These were established by James Thomson and colleagues at the University of Wisconsin, Madison, in 1998 (Thomson et al. 1998) and their availability has expanded the range of human/non-human chimeras that scientists are capable of making. For example: