Typically derived from in vitro fertilized eggs destined for medical waste, ESCs are obtained from the inner cell mass(ICM) of the blastocyst. These cells are isolated before a portion of the ICM differentiates (post fertilization: 3.5 days in mice, 5 days in humans). With the proper signals, the cells can proliferate indefinitely without differentiating, even in vitro. These pluripotent cells can differentiate into ectoderm, mesoderm, or endoderm. Because of the ethical ramifications of their derivation, there is some controversy over the use of ESCs. However, they represent the most flexible and potent cells compared to stem cells obtained by other means. Given their unlimited potential, ESCs are a favorite among scientists in clinical and research settings.