ABCD1 and X-linked disease: Adrenoleukodystrophy (ALD), the most common PD, is an X-linked inherited disease caused by a pathogenic mutation in the ABCD1 gene and a dysfunction of its product, ABCD1, a peroxisomal membrane protein involved in the import of saturated very-long-chain fatty acids (VLCFA) into the peroxisomes, leading to impaired β-oxidation of saturated VLCFA.