As MASTL is a candidate target for radiosensitization in non-small cell lung cancer cells (19) and MASTL depletion inhibits the radioresistant breast cancer stem cells (BCSCs) through caspase-2 activation, which is a major caspase in the mitotic catastrophe in response to DNA damage (14), we examined the effects of MKI-1 on the radiosensitivity of breast cancer cells. Here, CASP2 is linked to breast cancer.