Defects in ACOX1 are the cause of adrenoleukodystrophy pseudoneonatal (Pseudo-NALD), also known as peroxisomal acyl-CoA oxidase deficiency. Pseudo-NALD is a peroxisomal single-enzyme disorder. Clinical features include mental retardation, leukodystrophy, seizures, mild hepatomegaly, hearing deficit. Pseudo-NALD is characterized by increased plasma levels of very-long chain fatty cids, due to decreased or absent peroxisome acyl-CoA oxidase activity. Peroxisomes are intact and functioning.