Every human being begins as an embryo consisting of a small bundle of stem cells—cells that are born as blank slates, and, when they receive the proper signals, turn into the skin, hair, muscle, blood, and other tissues that make up the human body. These toti- or pluripotent stem cells start with the ability to become any type of cell, but as the human body develops, they take on specific tasks that narrow their ability to differentiate. Five days after a fertilized egg gives rise to an embryo, the embryo’s inner mass includes embryonic stem cells (ESCs), which can become any type of cell. As stem cells reach their adult or postnatal state, they are no longer toti- or pluripotent, but may still differentiate into several types of specialized cells.