USP7 and breast cancer: ZEB1 was identified to be phosphorylated and stabilized by ATM after ionizing radiation treatment in breast cancer cells, and correspondingly, the upregulation of ZEB1 was proven to stabilize checkpoint kinase 1 (CHK1) by activating the deubiquitylation of ubiquitin-specific-processing protease 7 (USP7), thus promoting homologous recombination-dependent DNA repair and resulting in radioresistance [43].