This study evaluated the prevalence of CLDN18.2 positivity, defined as ≥ 75% of tumor cells demonstrating moderate-to-strong membranous CLDN18 staining using the VENTANA CLDN18 (43-14A) RxDx Assay (for Investigational Use Only; VMSI/Roche), and the association of CLDN18.2 status with demographic, clinical, and histopathological characteristics in tumors from patients with HER2-negative, LA unresectable or mG/GEJ adenocarcinoma. The gene discussed is CLDN18; the disease is neoplasm.