Embryonic stem cells or ES cells are undifferentiated cells derived from the inner cell mass of 3 to 5 day old mammalian embryos. Human ES cells may be sourced from spare embryos from an in vitro fertilization procedure. When grown in a culture medium, these cells can renew themselves indefinitely without losing their ability to differentiate. ES cells are pluripotent, meaning they can transform into any cell type of the body. By providing specific growth factors, the cells can transform into terminally differentiated cells, such as muscle cells, neurons, or pancreatic beta cells. Additionally, ES cells are also used to screen drugs, diagnose genetic disorders, or repair tissue damage caused by injury or aging.