CASP6 and neoplasm: First, PDT can activate caspase-8 and caspase-9, and then the active form of caspase-8 and caspase-9 initiates caspase-3 and caspase-6 activation to subsequently activate the remainder of the caspase cascade, which degrades PARP, and finally leads to apoptosis of tumor cells [25], [26].