TPH1 and major depressive disorder: Tryptophan 5-hydroxylase 1 (TPH1) is the rate-limiting enzyme in 5-HT biosynthesis and alterations in this enzyme have been implicated in depression (Gizatullin et al., 2006), suicide attempts (Galfalvy et al., 2009), deliberate self-harm (Pooley et al., 2003) and the antidepressant-treatment response (Ham et al., 2007).