Figure 1: Barriers to the use of human stem cells for drug development and cellular therapy Human embryonic stem cells (ESCs), distributed stem cells (DSCs), and induced pluripotent stem cells (iPSCs) are derived from human tissues at the indicated stages of human development. Related to the differences in their method of derivation, the three types of human stem cells have specific profiles of barriers (Note: the height of red bricks indicates the degree of difficulty) that must be overcome (curved arrows) to achieve their successful development of applications for drug evaluation and cellular transplantation therapies.