A transplant is an organ, tissue, or a group of cells removed from one person (the donor) and transplanted into another person (the recipient) or moved from one site to another in the same person. A skin graft is a common example of a transplant from one part of a person's body to another part. Tissue or cell transplantation is an invaluable technique with a multitude of applications including studying the developmental potential of certain cell populations, dissecting cell-environment interactions, and identifying stem cells. One key technical requirement for performing transplantation assays is the capability of distinguishing the transplanted donor cells from the endogenous host cells and tracing the donor cells over time