We describe a new autosomal recessive white matter disorder ('hypomyelination and congenital cataract') characterized by hypomyelination of the central and peripheral nervous system, progressive neurological impairment and congenital cataract. We identified mutations in five affected families, resulting in a deficiency of hyccin, a newly identified 521-amino acid membrane protein. Our study highlights the essential role of hyccin in central and peripheral myelination.