This may relate to metabolic aberrations occurring in cell types in which there is no co-expression of ABCD2 with ABCD1. X-ALD is the most frequently occurring peroxisomal disorder and characterized by the accumulation of very long-chain fatty acids (VLCFAs) in the brain, adrenal glands, and plasma (Kemp et al., 2012). The gene discussed is ABCD2; the disease is X-linked adrenoleukodystrophy.