The gold standard for ascertaining whether non-human cells are pluripotent involves injecting them into blastocyst-stage embryos. The resulting chimeras are then bred and the offspring examined to see if all of their cells are progeny of the inserted cells. If so, then the inserted cells were pluripotent, as evidenced by the fact that they could give rise to germ cells and, consequently, to an entire individual. In the case of hESCs, such a procedure would involve the creation of human offspring as a means for testing cell potency, not an ethically viable procedure. Instead, the standard method for ascertaining whether putative hESCs are truly pluripotent is to inject them into postnatal, immune-deficient mice, and see whether they give rise to teratomas (tumors that consist of disorganized tissue growth of the three embryonic germ layers, endoderm, mesoderm, and ectoderm).