GPI-anchored cell surface protein that regulates T- lymphocytes proliferation, differentiation, and activation. Regulates the T-cell receptor (TCR) signaling by interacting with component CD3Z/CD247 at the plasma membrane, leading to CD3Z/CD247 phosphorylation modulation. Restricts the entry of human coronaviruses, including SARS-CoV, MERS-CoV and SARS-CoV-2, by interfering with spike protein-mediated membrane fusion. Also plays an essential role in placenta formation by acting as the main receptor for syncytin-A (SynA). Therefore, participates in the normal fusion of syncytiotrophoblast layer I (SynT- I) and in the proper morphogenesis of both fetal and maternal vasculatures within the placenta. May also act as a modulator of nicotinic acetylcholine receptors (nAChRs) activity.