Monitoring of inflammatory biomarkers has played a crucial role in distinguishing between bipolar disorder and MDD, identifying significant differences in biomarker levels between these conditions.66,72 Markers such as IL-9, TNF-α, C-C motif chemokine ligand (CCL)3, CCL4, CCL5, CCL11, CCL25, CCL27, C-X-C motif chemokine ligand (CXCL)6 and CXCL11 have been indicative of bipolar disorder, whereas IL-1β, IL-2, IL-4, IL-6, IL-7, IL-10, IL-16, CCL20, CCL21, CXCL12, platelet-derived growth factor subunit B and vascular endothelial growth factor are more frequently associated with MDD. The gene discussed is CCL21; the disease is major depressive disorder.