Porcine endogenous retrovirus (PERV) is a retrovirus found in the DNA of all pigs. The virus is normally in an inactive proviral form, but can potentially produce mature viral particles infectious for other species. PERV infection is a risk of pig to human transplants. Porcine endogenous retroviruses (PERVs) are a significant obstacle to xenotransplantation because they can infect human cells in vitro and have the potential for transmission of unexpected pathogens to humans.