Porcine nucleic acid sequences flanking potentially infectious porcine endogenous retroviral (PERV) insertion sites have been identified and isolated. The unique flanking sequences include porcine nucleic acid sequences that flank the 3 end and porcine nucleic acid sequences that flank the 5 end of PERV insertion sites. The present invention provides compositions and methods for detecting presence of PERV in a sample, particularly those with infectious potential. In addition, the invention relates to breeding of pigs or selection of porcine tissue that is free of infectious PERV for use as a xenotransplant tissue.