Table 2.

Human endogenous retrovirus families1

	Copy number	Also found in	Biological roles
Class I
HERV‐E	35–50	Apes and Old World monkeys	Salivary amylase gene expression2
HERV‐H	50–100 intact, ∼900 deleted	Apes and Old World monkeys	LTRs involved in gene regulation in placenta3
HERV‐I	∼25	Apes and Old World monkeys	LTR may help regulate cytochrome c1 gene4
HERV‐P	10–20 each of three types	Apes, OW and NW monkeys	Not known
HERV‐R	1	Apes and Old World monkeys	Immunosuppression in placenta (env)5
HERV‐W	Multiple copies	Apes and Old World monkeys?	env gene product involved in placental morphogenesis6
ERV‐9	30–40; at least 4,000 LTRs	Apes and Old World monkeys	LTRs may be involved in gene regulation7
Class II
HERV‐K	30–50 intact, ∼ 25,000 solitary LTRs	Apes and OW monkeys; some are human‐specific	LTRs may be involved in gene regulation; expressed in placenta8

¹Class I, more closely related to murine leukemia virus; class II, related to mouse mammary tumor virus. Families defined by type of tRNA that binds to 5′ LTR primer binding site. General references: Shih et al., 1991; Wilkinson et al., 1994; Löwer et al., 1996. NW, New World; OW, Old World.

²Samuelson et al., 1990; Ting et al., 1992.

³Goodchild et al., 1992.

⁴Suzuki et al., 1990.

⁵Venables et al., 1995.

⁶Blond et al., 1999; Mi et al., 2000.

⁷Costas and Naveira, 2000.

⁸Akopov et al., 1998; Kapitonov and Jurka, 1999; Baust et al., 2000.