Then, we found that the expression of PD-1 was strongly correlated with T cells, CD8+ T cells, and cytotoxic lymphocytes in more than 80% of the cancer types (R ≥ 0.7), while the correlation between the expression of PD-1 and the neutrophils infiltration was the poorest among all types of the immune cells (Fig. 2b, Additional file 1: Figure S1). This evidence concerns the gene PDCD1 and cancer.