FIG4 and neuropathy: CMT4J is caused by a partial loss-of-function mutation on FIG4 phosphoinositide 5-phosphatase (FIG4), a ubiquitously expressed phosphoinositide 5 phosphatase, dephosphorylating PI3,5P2 to PI3P. CMT4J is a neuropathy with myelin defects and axonal loss in the periphery caused by a compound heterozygotic combination of a missense allele with a null allele.