Human Endogenous Retrovirus-W (HERV-W) is the coding for a protein that would normally be part of the envelope of one family of Human Endogenous Retro-Viruses, or HERVs.
HERV-W encoding sequences makes up about 1% of the human genome and are part of a superfamily of repetitive and transposable elements. There are 31 different families of HERVs that together make up about 8% of the human genome, this is four times more DNA than is devoted to protein coding genes.[1][2]
Most HERVs in the genome today are not replication-competent due to frame shifts, premature stop codons and recombination in their long terminal repeats (LTRs).[3] Each HERV family is derived from a single infection of the germline by an exogenous retrovirus that, once integrated, expanded and evolved.[4] A complete HERV contains U3RU5- gag- pro- pol- env –U3RU5, where U3RU5 are long terminal repeats (LTRs) and gag, pro, pol and env are genes.[clarification needed]
It is common for viruses to take pieces of their host's genome with them which can aid their success. On the other hand, hosts can also keep viral DNA in their genome which may persist if advantageous or non-deleterious. In the case of HERVs, viral DNA integrated into the germ-line genome of a human ancestor.[3] Thus, all the progeny of the infected human ancestor would have this viral genome integrated into every cell in their bodies.[3]
This new retroviral DNA can now be passed on vertically from parents to child.[3] Furthermore, the integrated viral genome has transposable element features, meaning it can replicate and/or jump in the human ancestor genome. Looking to the genomes of many species related to humans helped determine how long ago this retroviral genome was integrated into the human ancestor.
Performing southern blots with primate blood samples and gag, pol and pro probes (from 100MSRV[clarification needed]) suggested HERV-W entered the genome of catarrhines over 23 million years ago.[5] Later, blood samples from various primates: hominoids, Old World monkeys, New World monkeys and prosimians were probed using a fluorescently labeled HERV-W element derived from the gorilla fosmid library.[6] Fluorescence in situ hybridization (FISH) revealed HERV-W elements in all the primate blood samples except the tupaia.[6]