The US FDA has licensed three separate classes of CPIs to treat distinct cancers, namely CTLA-4 inhibitors (Ipilimumab), PDL-1 inhibitors (Avelumab, Durvalumab, and Atezolimumab), and PD-1 inhibitors (Cemiplimab, Pembrolizumab, and Nivolumab) [176]. Here, CTLA4 is linked to cancer.