We examined the levels of 5-HT, the expression of TPH protein and mRNA, their immunohistochemical localization, and TPH methylation to assess this hypothesis and determine whether 1) TPH1/2 expression was associated with depression and 2) the level of the TPH1/2 proteins showed the same trend as that of mRNAs transcription from the TPH1/2 gene. The gene discussed is TPH1; the disease is major depressive disorder.