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Orthostatic hypotension is a common clinical problem, but the underlying mechanisms have not been fully delineated.We describe 2 families, with 4 patients in total, experiencing severe life-threatening orthostatic hypotension because of a novel cause.As in dopamine β-hydroxylase deficiency, concentrations of norepinephrine and epinephrine in the patients were low. Plasma dopamine β-hydroxylase activity, however, was normal, and the DBH gene had no mutations. Molecular genetic analysis was performed to determine the underlying genetic cause. Homozygosity mapping and exome and Sanger sequencing revealed pathogenic homozygous mutations in the gene encoding cytochrome b561 (CYB561); a missense variant c.262G>A, p.Gly88Arg in exon 3 in the Dutch family and a nonsense mutation (c.131G>A, p.Trp44*) in exon 2 in the American family. Expression of CYB561 was investigated using RNA from different human adult and fetal tissues, transcription of RNA into cDNA, and real-time quantitative polymerase chain reaction. The CYB561 gene was found to be expressed in many human tissues, in particular the brain. The CYB561 protein defect leads to a shortage of ascorbate inside the catecholamine secretory vesicles leading to a functional dopamine β-hydroxylase deficiency. The concentration of the catecholamines and downstream metabolites was measured in brain and adrenal tissue of 6 CYB561 knockout mice (reporter-tagged deletion allele [post-Cre], genetic background C57BL/6NTac). The concentration of norepinephrine and normetanephrine was decreased in whole-brain homogenates of the CYB561(−/−) mice compared with wild-type mice (P<0.01), and the concentration of normetanephrine and metanephrine was decreased in adrenal glands (P<0.01), recapitulating the clinical phenotype. The patients responded favorably to treatment with L-dihydroxyphenylserine, which can be converted directly to norepinephrine.This study is the first to implicate cytochrome b561 in disease by showing that pathogenic mutations in CYB561 cause an as yet unknown disease in neurotransmitter metabolism causing orthostatic hypotension.