PDCD1 and nasopharyngeal carcinoma: PD-L1 expression was positively associated with PD-1 (r = 0.219, P <0.001), p-S6 (r = 0.273, P < 0.001) or co-expression of PD-L1 and PD-1 (r=0.366, P <0.001) in NPC.