CHEK1 and breast cancer: analyzed the protein kinase profiles of MCF-7 cell line and its cognate radiation-resistant cell line MCF-7/C6 and validated the results showed that checkpoint kinase 1 (CHK1), cell cycle protein-dependent kinases 1 and 2 (CDK1 and CDK2) were significantly upregulated in the radiation resistant cell lines; this suggests that both DNA repair and cell cycle were involved in regulating breast cancer radiation resistance (69).