CMT4B1 is caused by autosomal recessive mutations in the MTMR2 gene.
CMT4B1 symptom onset usually occurs by 3 years old. CMTers who have CMT4B1 usually experience difficulty with walking as the first clinical signs of their CMT. Progression is usually rapid with most requiring mobility assistance by their early 20’s.
CMTers who have CMT4B1 will usually develop facial and bulbar impairment, and CMTers who have CMT4B1 will usually develop diaphragm weakness resulting in respiratory impairment. Sensory responses are usually absent in nerve conduction study, and nerve conduction velocity is usually significantly slowed to less than 20 meters/sec (normal is considered ~50 meters/sec).