Effects of Probiotics as Adjunctive Therapy to Fluoxetine on Depression Severity and Serum Brain‐Derived Neurotrophic Factor, Cortisol, and Adrenocorticotropic Hormone in Patients With Major Depressive Disorder: A Randomized, Double‐Blind, Placebo‐Controlled Trial. The gene discussed is BDNF; the disease is major depressive disorder.