For predicting effectiveness of anti-PD-1/PD-L1 monotherapy, this open-label phase II study (ONO-4538-88) evaluated the potential of the tumor-infiltrating lymphocyte (TIL) biomarker: the balance between cytotoxic T cells and regulatory T cells.<h4>Patients and methods</h4>Patients with advanced non-small cell lung cancer (NSCLC) or gastric cancer were screened between March 2021 and January 2022. The gene discussed is CD274; the disease is gastric cancer.