Endogenous retroviruses (ERVs) are viral elements found in the genome of vertebrates that closely resemble (and can be derived from) retroviruses. In humans (where they are known as HERVs) this 'rogue' DNA is believed to make up between 5 - 8% of the human genome.

It's suggested that the rogue DNA was incorporated into the (pre-human) genome hundreds of millions of years ago. It's apparently mostly inactive, though some fragments have the potential to be be reactivated by new viral infections. Some fragments can code for proteins that seem to have no role in the human body - though recent research has suggested some might affect embryo development and disease proliferation.

What do these efficient genomic colonizers do? Are they merely fossils that, like mosquitos in amber, were stuck and preserved in large host genomes while their functions decayed? Researchers have been struggling to understand their roles for as long as we have known them, postulating junk, bystander, and pathogen hypotheses."

See : Roles of Endogenous Retroviruses in Early Life Events ^{[ paywalled ]}Trends in Microbiology, Volume 25, Issue 11 pp. 876-877

A 2024 genetic analysis project reinforced previous suggestions that HERVs may play some role in the occurrence of some severe mental disorders - such as Schizophreniaplugin-autotooltip__plain plugin-autotooltip_bigSchizophrenia

"Schizophrenia is a mental disorder characterized by abnormal social behavior and failure to understand what is real. Common symptoms include false beliefs, unclear or confused thinking, auditory and visual hallucinations, reduced social engagement and emotional expression, and a lack of motivation." and Attention-Deficit / Hyperactivity Disorder (ADHD)plugin-autotooltip__plain plugin-autotooltip_bigAttention-Deficit / Hyperactivity Disorder (ADHD)

Attention Deficit Hyperactivity Disorder (ADHD) is a neuro-developmental mental disorder. It's characterized by problems in paying attention, excessive activity, or difficulty in controlling behavior (typically not appropriate for a person's age). Symptoms usually begin at between six to twelve years of age, and, to be classified as ADHD must be present for more than six months and cause problems in at least two settings (e.g. school, home, or i… - but how this might work in practice is still completely unknown.

Human endogenous retroviruses (HERVs) are repetitive elements previously implicated in major psychiatric conditions, but their role in aetiology remains unclear."

Source : Nature Communications volume 15, Article number: 3803