PD-1, which binds to PD-L1, also known as CD274, or to PD-L2/CD273, is a peripheral immune checkpoint of immune, tumor, and stromal cells, whereas CTLA-4 binds to CD80/CD86, also known as B7-1/B7-2 co-stimulatory receptors, on antigen-presenting cells (APCs) [65]. Here, CTLA4 is linked to neoplasm.