Embryonic stem cells, or ES cells, are cells that can be isolated from early embryos, before they differentiate into specific types of cells. Because stem cells have the potential to generate fresh, healthy cells of nearly any type, there is interest in exploring their use to treat and cure various diseases. The societal controversy regarding human ES cells relates primarily to their derivation from very early embryos. In addition, certain stem cell lines are developed using a cloning technique called somatic cell nuclear transfer, which can generate cells that are an exact genetic match to a patient. From the Series:HHMI’s Understanding Biomedical Research Series