PD1, or programmed cell death protein 1, is a cell surface receptor protein encoded in humans by the gene, PDCD1. It may also be known as CD279, hPD-1, SLEB2, and PD-1. The full length protein is 288 amino acids long with a mass of 31.6 kDa, and is expressed in T cells, B cells, and macrophages. It functions in regulating the immune system by downregulating T cell activation. For this role, inhibitors that block PD1 have been studied as a means of activating the immune system to attack cancer cells.