To assess the radiosensitization potential of the ATR inhibition in different HNSCC subgroups, a clonogenic assay in three HPV-negative (SQD9, CAL27, and SCC61) and three HPV-positive (SCC154, SCC104, and SCC47) HNSCC cells was performed. The gene discussed is ATR; the disease is head and neck squamous cell carcinoma.