CHEK1 and breast cancer: This is called ATR-CHK1-Cell Division Cyclin 25A or cell division cyclin 25A (CDC25A) pathway.18,19 When ATR detects DNA double-strand breaks or long single-strand breaks, upstream ATR transmits DNA damage signals to CHK1 through mediators for activation.20 Activation of CHK1 inactivates CDC25A phosphatase, leading to cycle depended kinase (CDK) inactivation and the cell cycle arrest in S/G2 or G2/M phases.21,22 So far, high expression of CHK1 has been detected in multiple human tumors, such as breast cancer, lung cancer, liver cancer, gastric cancer, and nasopharyngeal cancer.