Interestingly, the expression level of PD-1 receptor controls the polarization of macrophages, i.e., a deficiency of PD-1 expression promoted the development of the pro-inflammatory M1 phenotype, whereas its overexpression enhanced the appearance of the immunosuppressive M2 phenotype, especially in tumor-associated macrophages (TAM) [51, 52]. The gene discussed is PDCD1; the disease is neoplasm.