As the first immune checkpoint explored for cancer immunotherapy, CTLA-4 was validated as an immunotherapeutic target after FDA approval of Ipilimumab for human use, either as monotherapy for melanoma, or as part of combination therapy with the anti-PD-1 antibody, Nivolumab, in melanoma, renal cancer, and colorectal cancer with microsatellite instability [1,2,3,4,5]. Here, CTLA4 is linked to cancer.