ES cells are established from embryo in a culture dish, and could differenciate into all cell types.� Once ES cells are injected into a blastcyst, they contribute to make� embryo.� The chimera is made of their own cells and ES cell-derived cells.�
Good ES cells could generate all the cell types in the chimera, while bad ES cells could not.� During the culture for establishment, almost all the ES cells carry arteficial random mutations, some of which cause cancer, leukamya, and other types of diseases.�
It is very important to check ES cell quality. Unfortunately, the best and the only way to check ES cells is to make chimera embryo.�