X-ALD is the most common peroxisomal disorder (affecting ~ 1:17,000 births), and is caused by mutations in the ABCD1 gene encoding the ALDP protein, an ABC half-transporter in the peroxisomal membrane facilitating the uptake of VLCFA for peroxisomal β-oxidation (Engelen et al. 2012) (Fig. 1). Here, ABCD1 is linked to peroxisomal disease.