An endogenous viral element (EVE) is a DNA sequence derived from a virus, and present within the germline of a non-viral organism. EVEs may be entire viral genomes (proviruses), or fragments of viral genomes. They arise when a viral DNA sequence becomes integrated into the genome of a germ cell that goes on to produce a viable organism. The newly established EVE can be inherited from one generation to the next as an allele in the host species, and may even reach fixation.
Endogenous retroviruses and other EVEs that occur as proviruses can potentially remain capable of producing infectious virus in their endogenous state. Replication of such 'active' endogenous viruses can lead to the proliferation of viral insertions in the germline. For most non-retroviral viruses, germline integration appears to be a rare, anomalous event, and the resulting EVEs are often only fragments of the parent virus genome. Such fragments are usually not capable of producing infectious virus, but may express protein or RNA and even cell surface receptors.