CTLA4 and esophageal cancer: Important immune checkpoint inhibitors are antibodies blocking programmed cell death ligand 1 (PD-L1) on the surface of tumor cells (expressed by 15–40% of esophageal cancers [26–28]), antibodies targeting programmed death-1 (PD-1) receptor on the surface of T cells and antibodies targeting cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) receptor on the surface of T cells.