Endogenous retroviruses are retroviruses thought to be derived from ancient infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and as such now remain in the genome. Retroviruses are viruses that reverse-transcribe their RNA into DNA for integration into the host's genome. Most retroviruses (such as HIV-1) infect somatic cells, but some can also infect germline cells and once they have done so and have been transmitted to the next generation, they are termed endogenous.