CD279 / PD-1 (programmed cell death 1), a transmembrane protein of CD28/CTLA-4 family. It is expressed inducibly mainly on activated T, B, and myeloid cells and plays a role in maintaining peripheral self-tolerance. Binding to its receptors CD273 and CD274 is associated with inhibition of T cell proliferation and induction of their anergy. It is also expressed during thymic development. Some variants of CD279 are associated with susceptibility to systemic lupus erythematosus, type 1 diabetes, and rheumatoid arthritis.