The techniques for culturing mouse embryonic stem (ES) cells from the inner cell mass of the preimplantation blastocyst were first reported 20 years ago , and versions of these standard procedures are used today in laboratories throughout the world. Additionally, studies of embryonal carcinoma (EC) cells from mice and humans have helped establish parameters for growing and assessing ES cells. It is striking that, to date, only three species of mammals have yielded long-term cultures of self-renewing ES cells: mice, monkeys, and humans (see Figure . Origins of Mouse Pluripotent Stem Cells).