TPH2 and Sudden unexpected death in epilepsy: Specifically, SUDEP patients presented with an alteration of the brainstem 5-HT system with a notable decrease in both tryptophan hydroxylase 2 (TPH2; rate-limiting enzyme in 5-HT synthesis) and the 5-HT presynaptic transporter (SERT; a protein that uptakes 5-HT from the synaptic cleft back to the presynaptic neurons) within the medullary raphe nuclei and/or the VLM13.