PDCD1 and cancer: While PD-1 receptor ligand PD-L1 has been shown to be expressed in various types of cancers [15], the immune inhibitory receptor PD-1 (also known as CD279 or PDCD1), a member of the extended CD28/CTLA-4 family, is known to be stably expressed only on T cells exposed to a chronic antigen [16, 17].