A total of 17 studies were identified, comparing DAT availability between people with MDD (n = 291) and controls (n = 392) (Amsterdam et al., 2012; Argyelan et al., 2005; Brunswick et al., 2003; Camardese et al., 2014; Hellwig et al., 2018; Hsiao et al., 2013; Hsieh et al., 2010; Laasonen-Balk et al., 1999; Lehto et al., 2006; Lehto et al., 2008b; Malison et al., 1998; Meyer et al., 2001; Moriya et al., 2020; Pizzagalli et al., 2019; Sarchiapone et al., 2006; Wu et al., 2011; Yang et al., 2008). This evidence concerns the gene SLC6A3 and major depressive disorder.