PDCD1 and neoplasm: Metronomic chemotherapy exerts immune-modulatory effects, unlike the standard chemotherapy regimen; for this reason, it was proposed to combine it with immunotherapies, such as anti-CTLA-4 and anti-PD-1, the immune checkpoint inhibitors [174], to enhance its effects in overcoming the cancer-induced immunosuppressive tumor microenvironment [31,34,175,176].