CASP6 and mucopolysaccharidosis type 4: Therefore, since the stimulation of apoptosis in cells affected by genetic defects, like Morquio disease, is assumed to proceed through the intrinsic pathway, and as we observed enhanced cytochrome c release from mitochondria in MPS IVA and MPS IVB cells (Figure 1), we investigated the levels of specifically cleaved caspases (caspase-9, caspase-3, caspase-6, and caspase-7) and PARP in Morquio and control cells, either untreated or treated with 1 μM staurosporine for 6 h.