ABCD1 and X-linked adrenoleukodystrophy: ALD (OMIM #300100) is the most frequently occurring peroxisomal disorder, and is caused by a defect in the ABCD1 gene, which encodes a transmembrane transporter protein that imports straight chain VLCFA-CoA esters into the peroxisome (Kemp et al 2012) (Fig. 1).