CCND1 and mantle cell lymphoma: We found that ataxia-telangiectasia mutated (ATM) deficiency, the second most common genomic alteration in MCL observed in 40%–50% of cases (36–38), exacerbated CCND1-induced RS, with accumulation of DNA damage in the mitotic phase of the cell cycle, increasing reliance of MCL cells on MMEJ.