The J43 monoclonal antibody specifically recognizes CD279 which is also known as PD-1 (programmed death-1). CD279 is a 50-55-kDa glycoprotein encoded by the Pdcd1 gene of the CD28 family of the Ig superfamily. The expression of Pdcd1 mRNA and PD-1 protein is tightly regulated. PD-1 is transiently expressed on CD4-CD8 thymocytes, it is upregulated on some cell lines upon induction of apoptosis, it is induced on thymocytes and splenic T and B lymphocytes after stimulation through their antigen receptors, and it is induced on activated myeloid cells. In addition, Pdcd1 mRNA is transiently expressed in developing B lymphocytes at the pro-B-cell stage. The presence of an ITIM (Immunoreceptor Tyrosine-based Inhibitory Motif) on PD-1's intracytoplasmic region and the development of splenomegaly and breakdown of peripheral tolerance in PD-1[-/-] mice suggest that PD-1 is involved in the negative regulation of immune responses. The PD-1 ligands, B7-H1 (also known as PD-L1, CD274) and B7-DC (PD-L2, CD273), are members of the B7 family of the Ig superfamily. The J43 antibody blocks the binding of PD-1 to its two ligands.