MEN1/ZES occurs in 20–25% of all ZES patients [15,86,126,165] and numerous studies have clearly established the fact that the presence of hyperparathyroidism in these patients (>95% at some point) [15,86,126,165,246] results in increased BAO, MAO, fasting gastrin levels, and increased resistance to antisecretory drug therapy [163,164,165,247,248]. The gene discussed is MEN1; the disease is Zollinger-Ellison syndrome.