Isolated pathogenic intragenic variants in <i>ABCD1</i> are associated with the most common peroxisomal disorder, X-linked adrenoleukodystrophy (X-ALD), a single transporter deficiency, which in its more severe cerebral form is characterised by childhood-onset neurodegeneration and high levels of very-long-chain fatty acids (VLCFA). The gene discussed is ABCD1; the disease is peroxisomal disease.