The concomitant presence of hyperparathyroidism, which is present in >95% of MEN1/ZES patients due to the parathyroid hyperplasia characteristically seen in MEN1 patients [5,314,343], can directly affect the behavior of the gastrinoma by stimulating the release of gastrin, increasing the basal acid output, affecting gastrin provocative secretin test positivity used in ZES diagnosis, and by increasing the difficulty in suppressing the acid secretion by medical therapy [33,251,254,315]. The gene discussed is GAST; the disease is multiple endocrine neoplasia type 1.