CLDN18 and gastroesophageal junction adenocarcinoma: CLDN18.2 expression was positive (defined as ≥ 75% of tumor cells showing moderate to strong membranous claudin 18 (CLDN18) staining) in 38% of patients with locally advanced unresectable or metastatic gastric or GEJ adenocarcinoma screened for participation in clinical trials [22, 23].