THE FIRST STEM cells were discovered in the early 1900s. Since then, stem cell research has mainly relied on three different sources for obtaining stem cells: embryonic cells, umbilical cord cells, and adult stem cells. For many years, murine embryonic stem cells have played a large part in helping us advance our understanding of many disease processes, including cancers. The abundance and ease of obtaining umbilical cord cells has also aided researchers.

Adult stem cells were used for the first time to produce a cloned mammal in 1995, when Dolly the sheep was born. However, it was not until 1998 that researchers took cells from human embryos and developed the first human embryonic stem cell lines. One of the first people to accomplish this feat was ...