Charcot-Marie-Tooth disease is a clinically and genetically heterogeneous disorder of the peripheral nervous system, characterized by progressive weakness and atrophy, initially of the peroneal muscles and later of the distal muscles of the arms. Classification CMT neuropathy is subdivided into CMT1 (see 118200) and CMT2 (see 118210) types on the basis of electrophysiologic and neuropathologic criteria. CMT1, or hereditary motor and sensory neuropathy type I (HMSN I), is a demyelinating neuropathy, whereas CMT2, or HMSN II, is an axonal neuropathy. Most patients with CMT are classified as having CMT1 or CMT2 by use of a cut-off value of 38 m/s for the motor median nerve conduction velocity (NCV). However, in some families with CMT, patients have motor median NCVs ranging from 25 to 45 m/s. Families of this type were reported by Salisachs (1974) and Davis et al. (1978). Davis et al. (1978) proposed that this form be designated 'intermediate' CMT. Claeys et al. (2009) stated that some CMT families may have an even broader range of NCV than 25 to 45 m/s, with the lowest levels around 25 and the highest levels within the normal range (50+ m/s). They also suggested that the term 'intermediate' should not be used to describe a single NCV value, but rather the CMT subtype at the level of the family (e.g., in families with a range or combinations of NCV values). Berciano et al. (2017) provided a detailed review of the different forms of intermediate CMT, noting that diagnoses may be controversial because of variable classification issues. The authors presented an algorithm for the interpretation of electrophysiologic studies in CMT, and suggested that nerve conduction studies should be conducted on the upper arm (axilla to elbow). They noted that distal axonal degeneration can result in secondary myelination defects, which may cause significantly decreased motor NCV and CMAP values that may be misinterpreted. Genetic Heterogeneity of Autosomal Dominant Intermediate CMT In addition to CMTDIB, which is caused by mutation in the DNM2 gene, other forms of dominant intermediate CMT include CMTDIA (620378), mapped to chromosome 10q24-q25; CMTDIC (608323), caused by mutation in the YARS gene (603623) on chromosome 1p35; CMTDID (607791), caused by mutation in the MPZ gene (159440) on chromosome 1q22; CMTDIE with focal segmental glomerulosclerosis (CMTDIE; 614455), caused by mutation in the INF2 gene (610982) on chromosome 14q32; CMTDIF (615185), caused by mutation in the GNB4 gene (610863) on chromosome 3q26; and CMTDIG (617882), caused by mutation in the NEFL gene (162280) on chromosome 8p21. [from OMIM]