Note: Supplementary information is available on the Nature Genetics website. ACKNOWLEDGMENTS We thank the clinicians and families for providing samples, N. Killeen for technical assistance and the Wellcome Trust for financial support.

A gene mutated in Charcot-Marie-Tooth disease type 4B (CMT4B), an autosomal recessive demyelinating neuropathy with myelin outfoldings, has been mapped on chromosome 11q22. Using a positional-cloning strategy, we identified in unrelated CMT4B patients mutations occurring in the gene MTMR2, encoding myotubularin-related protein-2, a dual specificity phosphatase (DSP).

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