One of the major stumbling blocks in xenotransplantation is the fear that transplanting animal organs, cells and tissues into people could expose them to an animal virus that could decimate not only the original recipient [who would be heavily immunosuppressed in the transplant setting], but could spread to the general population. Since xenotransplantation efforts are currently focused on using pigs as donors, many scientists have targeted their research on a virus native to every pig cell--the porcine endogenous retrovirus (PERV). Some worry, however, that in focusing on the PERV threat, scientists may be missing still other, unknown viruses.