PDCD1 and neoplasm: As shown in Fig. 1D, high PD-1 expression was associated with short overall survival (χ2 = 35.42, P < 0.001) and short disease-free survival (χ2 = 40.39, P < 0.001), suggesting that high expression of PD-1 on tumour-infiltrating CD8+ T cells was associated with poor prognosis.