All patients with G/GEJ adenocarcinoma should be tested at diagnosis for the following panel of biomarkers: HER2, MMR deficiency/MSI, CLDN18 expression, and PD-L1 expression.Optimally, testing for all predictive biomarkers should be carried out reflexively in the same laboratory, concurrently, if possible.NTRK fusions appear rare in G/GEJ adenocarcinomas. Depending on laboratory preference, the test may be included as part of reflex testing or may be requested at the discretion of the treating oncologist when third-line therapy or beyond is being considered. The gene discussed is CLDN18; the disease is hyperinsulinemic hypoglycemia, familial, 4.