HSD17B4 and X-linked adrenoleukodystrophy: In addition to the most prevalent disorder, X-linked adrenoleukodystrophy (X-ALD), which is evoked by mutations in the peroxisomal fatty acid transporter ABCD1 (Fig. 1), important SEDs with a severe brain pathology are caused by mutations in the genes of acyl-CoA oxidase 1 (ACOX1) and the multifunctional protein 2 (MFP2, encoded by HSD17B4) (Berger et al. 2016).