CD317, also known as BST2, Tetherin, and HM1.24, is a type II transmembrane GPI-protein with a molecular weight of about 29-33 kD. It is an interferon-induced protein expressed on dendritic cells, plasma cells, B lymphoblast cells, monocytes, granulocytes, T cells, NK cells, stromal cells, and some non-hematopoietic cells. BST2 inhibits cytokine production through interaction with ILT7 (CD85g). It is also involved in the regulation of B cell growth. More importantly, BST2 has been found to restrict the release of a number of viruses from infected cells, including all tested retroviruses (such as HIV-1) and some arenaviruses and filoviruses. In HIV-1 studies, it has been reported that BST2 retains the nascent virons on the surface of infected cells by incorporation of the protein into HIV-1 particles. HIV-1 Vpu is able to induce BST2 degradation.