CHEK1 and head and neck squamous cell carcinoma: It has been previously observed that both HPV-positive and HPV-negative HNSCC cells (UMSCC-47, UDSCC-2, UTSCC-45, 93-VU-147T, FaDu and Cal33) display increased sensitivity to X-ray irradiation following pre-treatment with the Chk1 inhibitor PF-00477736, in conjunction with an abrogation of the radiation-induced G2/M arrest [9].