The peroxisomal disorders are a clinically diverse group of genetic diseases.2- 5 This clinical diversity reflects the metabolic diversity of the enzymes contained in peroxisomes and the complex mechanisms required for the biosynthesis and importation of these enzymes into a peroxisome. The peroxisomal disorders are classified into 2 groups according to their morphologic appearance and the number of peroxisomal enzymes affected: peroxisomal assembly defects and single-enzyme defects. The peroxisomal assembly disorders are characterized by absent or reduced numbers of peroxisomes and multiple enzyme deficiencies and are caused by defects in the importation of peroxisomal proteins from the cytosol into the peroxisomal matrix. This group of disorders comprises 5 clinically defined disorders, including Zellweger syndrome, NALD, and infantile Refsum disease. Zellweger syndrome, the most severe disorder in the spectrum, is characterized by a prenatal onset, craniofacial dysmorphism, cataracts, retinopathy, hepatocellular disease, renal cysts, adrenal hypoplasia, chondrodysplasia punctata, profound hypotonia, a neuronal migration defect, severe psychomotor retardation, and leukodystrophy; death usually occurs in the first year of life. Patients with NALD and those with infantile Refsum disease have a similar but milder clinical course; they do not have renal malformations or chondrodysplasia punctata, and they may survive into childhood or, rarely, adulthood. When a patient has evidence of leukodystrophy, the diagnosis of NALD is made.4 Detailed genetic studies, including complementation analysis, have shown that the peroxisomal assembly disorders are genetically heterogenous, ie, the same phenotype may be caused by mutations of different loci. Conversely, mutations of a specific locus may produce different phenotypes. Complementation studies using direct gene transfer are currently under way to define the molecular basis of this complicated genotype-phenotype relationship.