Noticeably, in the two tested cancer cell lines, although the timing of caspase activation and Noxa up-regulation were different from each other, Noxa induction was assuredly observed earlier than the caspase activation, which implied that under the treatment of ABT-737, Celastrol or the combination, Noxa increment was triggered prior to the apoptotic cell death, raising the possibility that Noxa induction played a role in the apoptosis elicited by ABT-737 plus Celastrol. This evidence concerns the gene PMAIP1 and cancer.