A xenograft, or heterograft, is the transplantation of an organ, tissue or cells from a donor to a recipient of a different species. The xenotransplantation of an organ from an animal like a pig in a human, may be possible, but is confronted by major risks of rejection and the transmission of viruses. In cancer research, xenografting consists in implanting malignant human cells in strains of mice bearing a specific genetic trait that considerably reduces the phenomenon of rejection. In particular, it permits evaluating the action of therapeutic anticancer molecules before performing clinical tests on human beings.