TPH1 and major depressive disorder: Other evidence is that Vitamin D induces the expression of the serotonin-synthesizing gene tryptophan hydroxylase 2 while repressing the expression of tryptophan hydroxylase 1, which plays a definite role in serotonin synthesis, establishing a thin link formation of serotonin and Vitamin D levels, thus fostering its supplementation might play a significant role in depression and its treatment [25,26].