Research has focused in large part on pig organs and tissues because their biochemical profile is similar to that of human organs. However, associated with all types of xenotransplantation are some significant problems, including hyperacute rejection, delayed rejection, and cell-based immune rejection. Additionally, novel infections may be transmitted from the donor to the recipient and spread to the human population. There is considerable debate over the pros and cons of xenotransplantation, and the US and the UK have already taken significant regulatory steps to foster this debate and to control clinical trials. Several companies are pursuing xenotransplantation in step with the regulatory frameworks, as the potential market for this procedure is believed to be significant.