A second study [201] dealt with the diagnosis of ZES in MEN1 patients (ZES/MEN1), which comprises 20–30% of all ZES cases [49,281,282,283,284,285,286,287,288,289] and is the most frequent F-pNET syndrome seen in MEN1 patients occurring in 20–61% (mean-54% [9 series]) [25,51,232,290]. Here, MEN1 is linked to Zollinger-Ellison syndrome.