It is important to highlight that beside ethical concerns, safety issues regarding human embryonic stem cell-based therapy are the main problem for their clinical use. The pluripotency of human embryonic stem cells is  a doubled-edged sword; the same plasticity that permits human embryonic stem cells to generate hundreds of different cell types also makes them difficult to control after in vivo transplantation. When undifferentiated human embryonic stem cells are transplanted, teratomas, tumors that contain all three germ layers, could develop. Studies have revealed that appearance of teratoma is between 33-100% in human embryonic stem cell-transplanted immunodeficient mice, depending on the implantation site, cell maturation, purity, and implantation techniques.