IAPP and pancreatic insulinoma: Peptides Nle3-VF and Nle3-LF (Fig. 1c) carrying N-methylations at Val18/Phe20 and at Leu17/Phe19, respectively, fully suppressed IAPP fibrillogenesis and cytotoxicity as determined by ThT binding in combination with the 3-[4,5 dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction assay in cultured rat insulinoma (RIN5fm) cells (Fig. 1d–g, Supplementary Fig. 2b, c, and Supplementary Fig. 3a, b).