SLC6A15 and depressive symptom measurement: Slc6a15 recognizes neutral amino acids such as proline (Broer et al. 2006), which can be further used in the biosynthesis of glutamate, a neurotransmitter implicated in depression pathophysiology and treatment (Pilc et al. 2013), and has recently emerged as a susceptibility gene for major depression (Kohli et al. 2011).