SLC6A15 and major depressive disorder: (3)The SLCs with, overall, the greatest expression in total are SLC6A15 (a neutral amino acid transporter [Pramod et al., 2013]), whose activity has been implicated in depression (Kohli et al., 2011), and SLC25A3 (a mitochondrial phosphate transporter [Palmieri, 2013]), while that least expressed in toto is SLC6A5 (glycine transporter).