TPH1 and schizophrenia: The studies identified report that treatment-resistance may be differentiated from treatment-responsive schizophrenia by predominance of the C allele of the T102C polymorphism of the 5HT2A gene (encoding the main excitatory serotonin receptors) [32, 33] and predominance of the C/A genotype for the A779C of the TPH1 gene (encoding the enzyme that catalyses the conversion of tryptophan to 5-HTP, serotonin’s precursor) [33].