GAST and gastrin-producing neuroendocrine tumor: With regards to MEN1-associated duodenal functioning gastrinomas, these are often multiple and associated with diffuse gastrin cell (G cell) hyperplasia (simple, linear, micronodular or macronodular neuroendocrine cell hyperplasia) and multicentric gastrin-producing microtumours (between 300 microns and 2 mm), which represent probable precursor lesions [167] (Figure 3D).