PER2 and breast cancer: Furthermore, as both aberrant Per2 expression [30] and Dox [31] have been shown to induce cellular senescence in response to genotoxic stress, we propose that the induction of apoptosis seen following the sensitization of MDA-MB-231 breast cancer cells to the cytotoxic effects of Dox may in part be mediated by mitotic catastrophe following cellular senescence.