PER3 and major depressive disorder: Peripheral blood-based gene expression profiling provides a potential, non-invasive method to identify the molecular markers associated with risk stratification and depression severity.This study aimed to investigate the gene expression of five candidate genes-<i>CD19</i>, <i>MTPAP</i>, <i>PER3</i>, <i>GAR1</i>, and <i>SLC25A26</i>-in the peripheral blood of drug-naïve patients with MDD and to evaluate their potential as diagnostic biomarkers.