ABCD1 encodes ALDP, a peroxisomal ATP-binding cassette transporter essential for very long-chain fatty acid (VLCFA) metabolism. ALDP functions as an ATP-dependent transporter of VLCFA-CoA from the cytosol into the peroxisomal lumen and possesses fatty acyl-CoA thioesterase activity that hydrolyzes VLCFA-CoA before transport 1. These functions are critical for peroxisomal β-oxidation of VLCFAs, regulating energy metabolism, myelination, mitochondrial oxidative phosphorylation, and inflammatory responses 12. ABCD1 mutations cause X-linked adrenoleukodystrophy (X-ALD), characterized by impaired VLCFA β-oxidation leading to pathogenic accumulation in the nervous system, adrenal cortex, and testis 13. X-ALD presents with striking phenotypic variability: childhood cerebral ALD (CCALD) with severe white-matter demyelination and neurodegeneration; adrenomyeloneuropathy (AMN) with progressive myelopathy in adulthood; and Addison-only presentations with adrenal insufficiency 24. Primary adrenal insufficiency affects 70% of ALD/AMN patients, likely from VLCFA-mediated disruption of adrenal cell membrane function and ACTH receptor activity 1. No genotype-phenotype correlation exists 3. Lentiviral gene therapy delivering functional ABCD1 has halted leukodystrophy progression in early cerebral ALD, though insertional oncogenesis remains a risk 56.