Infantile Refsum disease is the mildest of a group of disorders known as peroxisome biogenesis disorders, Zellweger syndrome spectrum (PBD-ZSS). PBD-ZSS is a group of inherited genetic disorders that damage the white matter of the brain and affect motor movements. Peroxisome biogenesis disorders, in turn, are part of a larger group of disorders called leukodystrophies. IRD can cause low muscle tone (hypotonia), retinitis pigmentosa (a visual impairment that can lead to blindness), developmental delay, sensorineural hearing loss, and liver dysfunction. IRD usually presents at birth or in infancy. Most individuals with IRD can achieve motor milestones, though they may be delayed, and most individuals can communicate with a few words or signs. Leukodystrophy with loss of acquired skills can occur at any age and may stabilize or progress. Peroxisome biogenesis disorders are caused by genetic changes in one of the PEX genes and are inherited in an autosomal recessive manner. Adult Refsum disease and infantile refsum disease are separate disorders caused by different genetic defects.