Bone Marrow Stromal Antigen 2 (BST2) is a single-pass type II membrane protein that belongs to the tetherin family. BST2 is predominantly expressed in the liver, lung, heart and placenta. BST2 is involved in the sorting of secreted proteins. BST2 is a human cellular protein which inhibits retrovirus infection by preventing the diffusion of virus particles after budding from infected cells. BST2 is initially discovered as an inhibitor to HIV-1 infection in the absence of Vpu, it has also been shown to inhibit the release of other viruses such as retroviruses, filoviruses, arenaviruses, and herpes viruses. BST2 may play a role in B-cell activation in rheumatoid arthritis.