Only BDNF (Carlberg et al., 2014; Tadić et al., 2014; Lieb et al., 2018; Wang et al., 2018b; Hsieh et al., 2019; Wagner et al., 2019), 5-HTT (Kang et al., 2013; Domschke et al., 2014; Okada et al., 2014; Iga et al., 2016), and HTR1B (Gassó et al., 2017; Wang et al., 2018a) were assessed in multiple MDD studies. This evidence concerns the gene BDNF and major depressive disorder.