ATM and breast cancer: The mechanisms behind chemo-resistance include: 1) upregulation of multidrug resistance (MDR) genes that effectively transport drugs out of the cell; 2) alteration of drug-metabolizing enzymes, such as those in the glutathione s-transferase family (GST); 3) escape from apoptosis and increased DNA repair due to mutated tumor suppressor genes [p53, breast cancer 1/2 (BRCA1/2), and ataxia telangiectasia mutated (ATM) genes] [2]; and 4) impairment of mitotic spindle checkpoint leading to resistance to microtubule inhibitors [6].