A step forward has come with the introduction of inducible caspase-9 (iC9) protein, fused to human caspase-9 and to modified FK506-binding protein, which has overcome several concerns associated with HSV-TK [130,131,132,133,134,135,136,137,138] (Figure 2B) and has shown an effect against GVHD in leukemia [134,136,137]. The gene discussed is CASP9; the disease is leukemia.