To confirm the results from the multiple linear regression analysis in patients with TRD, baseline plasma BDNF concentration was compared in TRD smokers (0.24 ng/mL) vs. TRD nonsmokers (0.31 ng/mL), and plasma BDNF concentration was not significantly different (U = 0.392; p = 0.149; Mann–Whitney test). The gene discussed is BDNF; the disease is treatment resistant depression.