FASLG and neoplasm: When antigen-specific CTLs are induced and go to the tumor site there are mechanisms in place that prevent the tumor cells from getting killed by CTLs, i.e., downregulation of activation receptors, co-stimulatory molecules, or HLA class I antigens recognized by CTLs; upregulation of co-inhibitory molecules like PD-L1 release of soluble factors that inhibit Th cells, CTLs, and APCs; and altered FAS-L expression on the tumor cells causing apoptosis resistance (80–83).