Levels of 5-HT in adipose tissues, liver, heart, and circulation were not suppressed by AIF1 deficiency, and serum levels of 5-hydroxyindoleacetic acid (5-HIAA), a major 5-HT metabolite, were unchanged (Supplementary Fig. 6a–c, e)—these findings argue against a role for peripheral 5-HT in the obesity resistance phenotype. The gene discussed is HTR5A; the disease is obesity due to melanocortin 4 receptor deficiency.