CHEK1 and cancer: Of note, decreased expression of the activated forms of ATM, DNA-PK, BRCA1, and Chk1/2 kinases (Figure 6) is inversely related to an increased expression of ABC-transporters (Figure 3), thereby illustrating the decreased intensity of DNA damage in Dox-treated OS cells due to an increased efflux of the chemotherapeutic agents (e.g., Dox) from cancer cells.