,77,78,79,80 The implications of better understanding Pdcd1 epigenetic regulation and chromatin dynamics in T cell differentiation and exhaustion are far-reaching because stable heritable epigenetic programming of exhausted CTLs in viral infections and tumors may limit targeting the PD-1/PDL-1 signaling pathway to reinvigorate these cells.64 This evidence concerns the gene PDCD1 and viral infectious disease.