SPP1 and major depressive disorder: Next, CD1C (OR = 0.23, 95%CI = 0.11–0.35), SPP1 (OR = 0.56, 95%CI = 0.34–0.79), CD3D (OR = 0.77, 95%CI = 0.58–0.96), and CAMKK2 (OR = 1.43, 95%CI = 1.23–1.62) determined the independent candidate diagnostic biomarkers for MDD (Fig. 3E).