Keeping the genome stable is a daunting task because each cell in the human body receives an average of 10,000 DNA lesions every day. DNA damages can be spontaneously created during cellular metabolisms such as DNA replication or can be generated by environmental sources such as UV irradiation (Sunlight) or chemical agents (Smoking or chemotherapy drugs). Cells developed many different DNA repair pathways to fix every specific type of DNA lesions protecting the cells against an accumulation of DNA mutations and cell death. To coordinate all the repair operation, three checkpoint kinases ATR, ATM, and DNA-PKcs are activated by DNA damage and determine which processes in the cells have to be turned on in order to repair the DNA.