Many human endogenous retrovirus K (HERV-K) proviruses are unique to humans

Background: Endogenous retroviruses contribute to the evolution of the host genome and can be associated with disease. Human endogenous retrovirus K ( HERV-K) is related to the mouse mammary tumor virus and is present in the g enomes of humans, apes and cercopithecoids (Old World monkeys). It is unkno wn how long ago in primate evolution the full-length HERV-K proviruses that are in the human genome today were formed. Results: Ten full-length HERV-K proviruses were cloned from the human genom e. Using provirus-specific probes, eight of the ten were found to be presen t in a genetically diverse set of humans but not in other extant hominoids. Intact preintegration sites for each of these eight proviruses were presen t in the apes, A ninth provirus was detected in the human, chimpanzee, bono bo and gorilla genomes, but not in the orang-utan genome. The tenth was fou nd only in humans, chimpanzees and bonobos, Complete sequencing of six of t he human-specific proviruses showed that full-length open reading frames fo r the retroviral protein precursors Gag-Pro-Pol or Env were each present in multiple proviruses. Conclusions: At least eight full-length HERV-K genomes that are in the huma n germline today integrated after humans diverged from chimpanzees. All of the viral open reading frames and cis-acting sequences necessary for HERV-K replication must have been intact during the recent time when these provir uses formed. Multiple full-length open reading frames for all HERV-K protei ns are present in the human genome today.