5-Hydroxy-l-tryptophan (5-HTP), as the immediate precursor in the biosynthesis of 5-hydroxy-tryptamine (5-HT), can inhibit insulin and glucagon secretion in non-diabetic islet donors and significantly increased the release of insulin in response to glucose in islets isolated from T2D donors (Bennet et al., 2015, (Das et al., 2004). The gene discussed is GCG; the disease is type 2 diabetes mellitus.