OLFM4 and major depressive disorder: The purposes of our current study included: (i) to determine whether serum levels of LRFN5 and OLFM4 were altered in drug-naive MDD patients; (ii) to observe their putative changes during antidepressants treatment; (iii) to explore the correlations between LRFN5/OLFM4 and clinical parameters; (iv) to assess whether these two molecules could be used as potential biomarkers for MDD and treatment responsiveness.