Peroxisomal disorders are caused by mutations in genes that are involved in peroxisome biogenesis or that encode the enzymes and transporter proteins (which take up the enzymes from the cytoplasm) of the peroxisome. Peroxisomal disorders are congenital disorders , and they range from relatively moderate to severe in nature. The Zellweger spectrum, for example, includes Zellweger syndrome, neonatal adrenoleukodystrophy (NALD), and infantile Refsum disease. Zellweger syndrome is characterized by complete absence or reduction in the number of peroxisomes. It is the most severe condition within the Zellweger syndrome. Mutations giving rise to Zellweger syndrome cause copper , iron , and substances called very long chain fatty acids to accumulate in the blood and in tissues, such as the liver , brain, and kidneys . Infants with Zellweger syndrome are often born with facial deformity and intellectual disability ; some may have impaired vision and hearing and may experience severe gastrointestinal bleeding or liver failure. Prognosis is poor: most infants with Zellweger syndrome do not live beyond one year. Symptoms of NALD and infantile Refsum disease, by contrast, appear in late infancy or in childhood, and patients may survive to early adulthood. Likewise, patients with RCDP may survive into childhood or, in mild cases, early adulthood.