TH and mood disorder: Most of the published genetic association studies of mood disorders have focused on functional polymorphisms in the loci encoding the serotonin transporter (SLC6A4), serotonin 2A receptor (5HTR2A), tyrosine hydroxylase (TH), tryptophan hydroxylase 1 (TPH1), and catechol-o-methyltransferase (COMT); however, recent meta-analysis of 183 papers that studied 393 polymorphisms in 102 genes found significant evidence for association for the following genes: APOE, DRD4, GNB3, MTHFR, SLC6A3 and SLC6A4 (Lopez-Leon et al. 2008).