This is elicited through interaction with the ligands PD-L1 and PD-L2.2 Normally, the expression of PD-1 is transient, and expression is downregulated during the resolution phase of the insult following clearing of antigen.2 However, in states of chronic antigen exposure, such as in cancer and autoimmunity, high PD-1 expression is sustained,3 and is typically associated with states of T cell dysfunction or exhaustion.4 The gene discussed is PDCD1; the disease is cancer.