CTLA4 and cancer: The action of HMAs against the leukemic cells is mediated by upregulation of tumor antigens expressed by the class 1 major histocompatibility complex and co-stimulatory molecule, resulting in the destruction of cancer cells, but eventual upregulation of program cell death protein (PD-1) and cytotoxic T- lymphocyte associated protein 4 (CTLA-4) leads to immune escape and drug resistance [69].