In 1998 a team of researchers reported that they had isolated and removed stem cells from the inner cell mass of human embryos that had been donated by couples undergoing fertility treatment (Thomson et al. 1998). The embryos had divided for several days to reach the blastocyst stage of approximately 100 cells. At this stage embryos have a hollow sphere in the middle, an outer layer of cells committed to forming the placenta and other cell lines, and a mass of undifferentiated cells pushed to one side (inner cell mass). The cells in the inner mass have, for a short time, the capacity to develop into all cells in the human body, and are known as embryonic stem (ES) cells. The researchers' announcement that they had isolated ES cells in human embryos generated considerable interest because it suggested that the cells could be removed, cultured, and coaxed...