CTLA4 and cancer: Ipilimumab, a recombinant human immunoglobulin (Ig) G1 monoclonal antibody targeting CTLA-4, in combination with nivolumab, is approved for the treatment of several cancers, and various trials are investigating PD-1/PD-L1-directed therapy in combination with drugs targeting other co-inhibitory and co-stimulatory immune checkpoints [52].