BST2 also known as CD317 is a single-pass type II membrane protein predominantly expressed in liver, heart, placenta, and lung tissues, as well as on synovial and bone marrow stromal cell lines. BST2 acts as a tethering factor for newly formed virions, playing a crucial role in innate immune response against viral infections. This function helps restrict virus release from infected cells, limiting viral spread and enhancing host control of infections. BST2 has also been implicated in B cell activation, especially in rheumatoid arthritis, highlighting BST2′s significance in immune regulation and potential therapeutic targeting.