The strongest evidence is at the level of the serotonin transported genes (mainly SLC6A4) with candidate genes studies suggesting an increased in methylation in depression (Kang et al., 2013; Philibert et al., 2008; Zhao, Goldberg, Bremner, & Vaccarino, 2013), BD (Sugawara et al., 2011) and reporting a positive association with symptoms severity (Olsson et al., 2010), comorbid depression in those with panic disorder (Schiele et al., 2019), and improvement from baseline to follow-up (Perez-Cornago, Mansego, Zulet, & Martinez, 2014). This evidence concerns the gene SLC6A4 and depressive disorder.