Programmed cell death protein 1, also called PD-1 or PDCD1 is a protein that in humans is encoded by the PDCD1 gene. This gene encodes a cell surface membrane protein of the immunoglobulin superfamily. By fluorescence in situ hybridization, the PDCD1 gene is mapped to 2q37.3. This protein is expressed in pro-B-cells and is thought to play a role in their differentiation. In mice, expression of this gene is induced in the thymus when anti-CD3 antibodies are injected and large numbers of thymocytes undergo apoptosis. Mice deficient for this gene bred on a BALB/c background developed dilated cardiomyopathy and died from congestive heart failure. These studies suggest that this gene product may also be important in T cell function and contribute to the prevention of autoimmune diseases.