A GEP-NET was the first clinical manifestation in 20 individuals (13.79% of all MEN1 affected patients; 23.26% of GEP-NET cases); 9 of them were gastrinomas (of which 4 were diagnosed after manifesting ZES, 2 after presenting duodenal ulcer and 3 after gastric symptoms), 8 were insulinomas (of which 4 were diagnosed after manifesting constant hypoglycaemia and/or recurrent hypoglycaemic crisis), one was VIPoma (initially diagnosed by elevated serum VIP level), one was PPoma (initially diagnosed by elevated serum PP and chromogranin A levels) and one was a gastric NF-NET. This evidence concerns the gene VIP and duodenal ulcer.