CD274 and Merkel cell skin cancer: Out of the target antigens discussed, three mAbs against PD-L1 have been approved by the US FDA which include durvalumab (IMFINZI®; approved for the treatment of bladder cancer in 2017), avelumab (Bavencio®; approved for the treatment of Merkel cell carcinoma in 2017), atezolizumab (Tecentriq®; approved for the treatment of bladder cancer in 2016) (156–158).