PDCD1 and melanoma: Furthermore, targeting of the cytotoxic T-lymphocyte-associated antigen (CTL4) immune checkpoint with ipilimumab (a human monoclonal antibody) improved overall survival of melanoma patients [3], and a combination of treatments that target the PD-1(Programmed cell death protein 1) checkpoint inhibitor resulted in more prolonged progression-free survival compared to ipilimumab alone [4].