CTLA4 and neoplasm: The generation of immune resistant tumor cells can occur in several ways: through loss of tumor antigens expression; through downregulation of MHC; through the overactivation of the prooncogenic transcription factor STAT3; through the overexpression of antiapoptotic effector BCL-2; through the expression of inhibitory cell surface molecules, such as programmed cell death 1 ligand 1 (PD-L1), cytotoxic T-lymphocyte associated protein-4 (CTLA-4), and Fas ligand (FasL), which directly kill cytotoxic CD8+ T cells.