CHEK1 and head and neck squamous cell carcinoma: In addition, cell cycle and DNA damage/repair related genes such as PARP1 (poly ADP-ribose polymerase 1), ATR (ATM and Rad3-related), XRCC3 (X-ray repair cross-complementing group 3), CDK5 (cyclin dependent kinase 5), and CHEK1 (checkpoint kinase 1) exhibited significantly reduced expression levels in CBD-treated HNSCC (Fig. 6B).