 This study seeks to: 1 replicate this study using PET scans in 20 subjects with MDD but extending it to patients with treatment-resistant depression TRD. Results from these participants with be compared to those from 10 non-depressed controls; 2 explore the correlation of the brain MAO-A level biomarker to treatment outcome by treating the 20 PET-imaged TRD patients with an MAOI, hypothesizing that their MAOI response will be related to their level of MAO-A. Brain MAO-A is an ideal candidate biomarker for this study since it appears to be significantly abnormally elevated in MDD, yet it has a broad range of values even among depressed patients. Most importantly, the MAO-A biomarker is known to be the single pharmacologic target of the treatment, making it appear likely that outcome with MAOI treatment will be related to MAO-A. 