Syncytin-2 also known as endogenous retrovirus group FRD member 1 is a protein that in humans is encoded by the ERVFRD-1

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

. This protein plays a key role in the implantation of human embryos in the womb. This gene is conserved among all primates, with an estimated age of 45 million years. The receptor for this fusogenic ''env'' protein is

MFSD2 Major facilitator superfamily domain-containing protein 2 (MFSD2 or MFSD2A) -- also known as sodium-dependent lysophosphatidylcholine symporter 1 -- is a protein that in humans is encoded by the ''MFSD2A'' gene. MFSD2A is a membrane transport pr ...

. The mouse syncytins are not true orthologues. The virus, along with some very similar insertions, belong to a group under the ''Gammaretrovirus''-like class I ERVs. Similar ERVs are found in

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

s, a result of an independent integration event. A proposed nomenclature suggests putting all such "class I" elements in a genus-level taxon separate from ''Gammaretrovirus''.

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