Immune checkpoint inhibitors (ICIs) function by blocking one or more surface protein interactions that regulate the adaptive immune response, chiefly the interaction between PD-L1 (tumor and stromal cells) and PD1 (T cells) that prevents tumor cell killing by cytotoxic T lymphocytes or between CTLA-4 (T cells) and CD80/CD86 (antigen-presenting cells) that leads to T cell anergy. The gene discussed is CTLA4; the disease is neoplasm.