VSIR and neoplasm: In addition to PD-1 on T cells and PD-L1 on tumor cells, N-linked glycosylation is a common occurrence in membrane-bound immune checkpoint proteins, such as B7 homolog 3 (B7-H3), V-set domain-containing T-cell activation inhibitor 1 (VTCN1; also known as B7-H4), V-domain immunoglobulin suppressor of T cell activation (VISTA; also known as B7-H5), and CD200 receptor 1 [100–103].