CASP9 and T-cell leukemia: In line with previously described arsenic drugs, As2-induced apoptosis is likely executed through the mitochondrial apoptosis pathway, as indicated by the loss of mitochondrial membrane potential, activation of caspase-9, insensitivity towards a lacking functional FADD adapter protein, and the rescue of apoptotic activity by Smac/DIABLO overexpression in T cell leukemia cells.