For EDU dyeing, HCT-116 cells and HCT-116 corrected clone cells were seeded into 96-well cell culture plates (5000 cells/well). After 24 hrs, 25 µM 5-ethynyl-2ʹ-deoxyuridine (EDU) was added for an additional 2 hrs at 37°C. Then, the cells stain with Cell-LightTM Apollo Stain Kit (RIBOBIO, Guangzhou, China) after washed with PBS and fixed with 4% formaldehyde for 30 mins. Before stained with Hoechst (RIBOBIO, Guangzhou, China) for 15mins, the cells were permeabilized with 0.5% Triton X-100 (Solarbio, Beijing, China) for 15 min. Images were obtained using a fluorescent microscopy (BX-51, Olympus Corporation, Tokyo, Japan).
For clonogenic assay, cells were plated onto 6-well plates (500 cells/well). Plated cells were cultured in 5% CO2 with 95% humidity at 37°C for 14 days. The plates were then fixed with 4% formaldehyde and stained with Giemsa. The colonies containing more than 50 cells were counted using ImageQuantTL7.0 Image Analysis Software (GE Healthcare, Buckinghamshire, UK).