In melanoma, flow cytometry was applied to study mechanisms of effects of immunotherapies, such as a PD-1 protein blockade by monoclonal antibodies pembrolizumab or nivolumab [196,197,198], CTLA-4 blockade with ipilimumab [198,199,200,201] or high-dose IL-2 therapy [202,203]. The gene discussed is PDCD1; the disease is melanoma.