Mitochondrial ribosomal protein L9 (MRPL9), mitochondrial ribosomal protein L15 (MRPL15), and mitochondrial ribosomal protein S2 (MRPS2) were the top three proteins potentially playing key roles in MDD; MRPL9 was the only protein with a cut-off value >0.75. Here, MRPL9 is linked to major depressive disorder.