CHEK1 and head and neck squamous cell carcinoma: In this study, we have characterised the radiosensitisation potential of both Chk1 and Wee1 checkpoint kinase inhibition (using MK-8776 and MK-1775, respectively) in response to X-rays and PBT (including an examination of both low and relatively high-LET protons) in 2D and 3D HNSCC models.