Studies have detected the expression of four genes encoding COX-2, MPO, iNOS, and secretory phospholipase A2 type IIA, namely PTGS2, MPO, NOS2A, and PLA2GA, and found that they are all elevated in patients with depression, highlighting the interrelationship between MDD and mRNA (Gałecki et al., 2012). Here, MPO is linked to depressive disorder.