PDCD1 and neoplasm: Immune checkpoint therapy targeting T cells, for instance, cytotoxic T lymphocyte-associated antigen 4 (CTLA4) and programmed cell death protein 1 (PD1) has shown promising results in patients with melanoma in recent years, but only some tumor types have benefited, the degree of tumor infiltration and immune cell activation, particularly CD8+ T cells, are both factors that influence immune checkpoint therapy efficacy (Kim et al., 2020).