SLC6A4 and schizophrenia: Furthermore, both the mRNA and protein levels of the 5-HT transporter (SERT) (p < 0.001 in mRNA level and p < 0.001 in protein level, Additional file 1: Fig. S2F, Fig. 2B, and Additional file 1: Table S1) and the 5-HT synthesis enzymes tryptophan hydroxylase-2 (TPH2) (p < 0.05 in mRNA and p < 0.001 in protein levels, Additional file 1: Fig. S2G, Fig. 2C, and Additional file 1: Table S2) were significantly lower in first-episode schizophrenia patients compared to the healthy controls.