CTLA4 and metastatic melanoma: Following the FDA approval of ipilimumab [an antibody targeting the cytotoxic T lymphocyte-associated protein 4 (CTLA4)] for treating metastatic melanoma in 2011 [8], immune checkpoint inhibitor therapy has emerged as a first-line immunotherapy for melanoma by targeting CTLA-4 and PD-1 on T cells or programmed cell death ligand 1 (PD-L1) on tumor cells to prevent anti-tumor responses [7,9].