Aldehyde oxidase (AO), a protein involved in the catabolism of catecholamines, is the product of a gene potentially responsible for one of the familial forms of the motor neuron disease, amyotrophic lateral sclerosis (ALS). Here, we report on the cloning of a partial cDNA coding for the mouse enzyme. Using this cDNA as a probe, we demonstrate that the AO transcript is expressed in the epithelial component of the choroid plexus. More importantly, in the gray matter, the mRNA is selectively localized in the large motor neurons of the nuclei of facial, motor trigemini and hypoglossus nerves and in the motor neurons of the anterior horns of the spinal cord. This localization is consistent with a possible role of AO in the pathogenesis of ALS.