Human endogenous retroviruses (HERV) comprise a significant part of the human genome, with approximately 98,000 ERV elements and fragments making up 5–8%. According to a study published in 2005, no HERVs capable of replication had been identified; all appeared to be defective, containing major deletions or nonsense mutations (not true for HERV-K). This is because most HERVs are merely traces of original viruses, having first integrated millions of years ago. An analysis of HERV integrations is ongoing as part of the 100,000 Genomes Project.
A 2023 study found HERV can become awakened from dormant states and contribute to aging which could be blocked by neutralizing antibodies.
Human endogenous retroviruses were originally discovered when human genomic libraries were screened under low-stringency conditions using either probes from animal retroviruses or by using oligonucleotides with similarity to virus sequences.