A genetic variant of the upstream regulatory region of the <i>SLC6A4</i> gene encoding SERT, the so-called short (s) allele, in comparison with the long (l) allele, results in the decreased function of this transporter, altered serotonergic regulation, an increased risk of psychiatric pathology and type-2 diabetes and obesity, especially in older women. Here, SLC6A4 is linked to type 2 diabetes mellitus.