During embryonic development, stem cells generate all the specialized cells that populate body tissues such as muscle, the nervous system, and blood. The term embryonic stem cells, or ES cells, is used by researchers for cells that can be isolated from early embryos, before they differentiate into specific types of cells. Depending on when they are isolated, embryonic stem cells are pluripotent-able to become virtually any type of cell—or multipotent—able to become many, but not all, 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.