Objective: To determine the phenotypic and cellular expression of two novel connexin32 (Cx32) mutations causing X- linked Charcot- Marie- Tooth disease (CMT1X). Methods: The authors evaluated several members of two families with CMT1X clinically, electrophysiologically, pathologically, and by genetic testing. The Cx32 mutations were expressed in vitro and studied by immunocytochemistry. Results: In both families, men were more severely affected than women with onset in the second decade of life. In the first family, the phenotype was that of demyelinating polyneuropathy with variable involvement of peripheral nerves. There was clinical evidence of CNS involvement in at least three of the patients, with extensor plantar responses and brisk reflexes. In the second family, the affected man presented with symmetric polyneuropathy and intermediate slowing of conduction velocities, whereas affected women had prominent asymmetric atrophy of the leg muscles. The authors identified two novel missense mutations resulting in L143P amino acid substitution in the first family and in V140E substitution in the second family, both located in the third transmembrane domain of Cx32. Expression of these Cx32 mutations in communication- incompetent HeLa cells and immunocytochemical analysis revealed that both mutants were retained intracellularly and were localized in the Golgi apparatus. In contrast to wild- type protein, they did not form gap junctions. Conclusion: These novel connexin32 (Cx32) mutations cause a spectrum of clinical manifestations characteristic of Charcot- Marie- Tooth disease (CMT1X), including demyelinating or intermediate polyneuropathy, which is often asymmetric, and CNS involvement in one family. The position and cellular expression of Cx32 mutations alone cannot fully predict these phenotypic variations in CMT1X. Objective: To determine the phenotypic and cellular expression of two novel connexin32 (Cx32) mutations causing X-linked Charcot-Marie-Tooth disease (CMT1X). Methods: The authors evaluated several members of two families with CMT1X clinically, electrophysiologically, pathologically, and by genetic testing. The Cx32 mutations were expressed in vitro and studied by immunocytochemistry. Results: In both families, the men were more severely affected than women with onset in the second decade of life. In the first family, the phenotype was that of demyelinating polyneuropathy with variable involvement of peripheral nerves. There was clinical evidence of CNS involvement at least three of the patients, with extensor plantar responses and brisk reflexes. In the second family, the affected man presented with symmetric polyneuropathy and intermediate slowing of conduction velocities, affected women had prominent asymmetric atrophy of the leg muscles. The authors identified two novel mi ssense mutations in the first family and in V140E substitution in the second family, both located in the third transmembrane domain of Cx32. Expression of these Cx32 mutations in communication- incompetent HeLa cells and immunocytochemical analysis revealed that both mutants were retained in the surface of the wild type protein, they did not form gap junctions. Conclusion: These novel connexin32 (Cx32) mutations cause a spectrum of clinical manifestations characteristic of Charcot-Marie-Tooth disease (CMT1X ), including demyelinating or intermediate polyneuropathy, which is often asymmetric, and CNS involvement in one family. The position and cellular expression of Cx32 mutations alone can not fully predict these phenotypic variations in CMT1X.