<h4>Background</h4>This multicentre, modular, Phase 1 study evaluated escalating doses of ATR (ataxia telangiectasia and Rad3-related kinase) inhibitor ceralasertib plus PD-L1 inhibitor durvalumab in patients with previously treated advanced/metastatic non-small-cell lung cancer (NSCLC) or head and neck squamous cell carcinoma (HNSCC).<h4>Methods</h4>Patients received ceralasertib 80/160/240 mg twice-daily (BID) or 320 mg once-daily (QD) for 7 (Days 22-28) or 14 (Days 15-28) days, plus durvalumab 1500 mg (Day 1), per 28-day cycle. The gene discussed is ATR; the disease is ataxia telangiectasia.