Mirtazapine D3 (Org3770 D3; 6-Azamianserin D3) is a deuterium labeled Mirtazapine. Mirtazapine is a 5-HT receptor inhibitor. Mirtazapine is a potent and orally active noradrenergic and specific serotonergic antidepressant (NaSSA) agent by blocking 5-HT2 and 5-HT3 receptors^[1]. In Vivo: Mirtazapine (intraperitoneal injection; 10–50 mg/Kg; 14 days) treatment normalizes heart rate, breath rate, anxiety levels and eliminates the hopping behavior observed in MeCP2-null mice, leading to improved phenotypic score^[2].