Defects in ABCD1 cause adrenoleukodystrophy, one of the most prominent peroxisomal single enzyme deficiencies (PEDs), which is caused by an accumulation of VLCFAs resulting in leukodystrophy and adrenal insufficiency (Engelen, 2024; Bougnères and Le Stunff, 2025; Vaz et al., 2025), whereas patients with a defective ABCD2 have not yet been identified. The gene discussed is ABCD1; the disease is X-linked adrenoleukodystrophy.