Charcot-Marie-Tooth disease type 4B3 (CMT4B3) is a subtype of Charcot-Marie-Tooth type 4 characterized by a childhood onset of slowly progressing, demyelinating sensorimotor neuropathy, focally folded myelin sheaths in nerve biopsy, reduced nerve conduction velocities (less than 38 m/s), and the typical CMT phenotype (i.e. distal muscle weakness and atrophy, and sensory loss).