During the early embryogenesis 5-HT neurons are generated from ventricular zone precursors controlled by fibroblast growth factors 4 and 8 (Fgf4 and Fgf8) and sonic hedgehog (Shh). In post-mitotic neurons, a complex network of genes such as Pet1, Lmx1b, Nkx2.2, Mash1, Gata2, Gata3, and Phox2b will determine the important proteins for 5-HT biosynthesis and neurotransmission such as Tph, Aadc, SERT. (for details see Fig. 1).