The process of transplanting a living tissue, cells, or organs from one species to another is known as xenotransplantation and, the tissue, cells, or organs transplanted are known as xenografts. Xenotransplantation is possibly most well known for the transplant of organs originating from animals such as baboons and pigs into humans. The first and possibly most famous case of a xenotransplantation is that of Fae, an infant girl who successfully received a baboon heart in 1984. Despite the potential future use of this method to replace organs, cell xenotransplantation is a much more common procedure utilized in cancer research. This process most commonly involves implantation of human tumor cells into immunodeficient mice and is used to test the efficiency of compounds and their interactions with pathways within the body. Because human tumor cells can relatively easily be injected into mice and tumor growth can be routinely established, these models are commonly used for testing compounds. In a paper published by Baselga et al. xenografts were used to determine the effects of two anticancer drugs paclitaxel and doxorubicin in combination with an anti-HER2 antibody on breast cancer cell growth [1]. We have used xenografts to test the tumor formation of the SUM149 cell line in SCID mice. Mice were given subcutaneous injections of two million cells suspended in matrigel into the posterior quadrant of animals’ abdomens. Tumors are observed 3 weeks after the implantation (Fig. 8.1a), and the histology reveals a well-defined poorly differentiated tumor (Fig. 8.1b, c).