CD279 (Programmed Cell Death Protein 1; PD-1) is a type I transmembrane protein belonging to the CD28/CTLA-4 family of immunoreceptors that mediate signals for regulating immune responses. Members of the CD28/CTLA-4 family have been shown to either promote T cell activation (CD28 and ICOS) or downregulate T cell activation (CTLA-4 and PD-1). CD279 is expressed on activated T cells, B cells, myeloid cells and on a subset of thymocytes. In vitro, ligation of CD279 inhibits TCR-mediated T cell proliferation and production of IL-1, IL-4, IL-10 and IFN-γ. In addition, CD279 ligation also inhibits BCR mediated signaling. CD279 deficient mice have a defect in peripheral tolerance and spontaneously develop autoimmune diseases.