Expression of TPH1 [23,24] and PAX4 (a critical transcription factor in EC cell specification) [23] is augmented in the duodenum and gastric antrum of obese subjects, while levels of plasma 5-HT and serum levels of the 5-HT metabolite, 5-HIAA, are associated with measures of human obesity [24,34]. This evidence concerns the gene PAX4 and obesity due to melanocortin 4 receptor deficiency.