GJB1 and Charcot-Marie-Tooth disease: [7] CMTX alone is the second most common form of hereditary motor and sensory neuropathy, accounting for up to 15% of all CMT cases.[8] CMTX type 1 (CMTX1), constituting 90% of CMTX cases,[9] is caused by mutations in gap junction beta 1 (GJB1) gene on chromosome Xq13.1.