CMT1A is caused by a duplication of the PMP22 gene. Instead of the normal 2 copies of the PMP22 gene, CMTers who have 1A have 3 copies. This gene is duplicated because it lives at a segment of chromosome 17 that is duplicated—17p11.2-p12.
CMT1A symptoms usually occur by the time the CMTer is in their late teens, but neuropathy can be detected by 5 years old, and the CMT1A hallmark slowed nerve conduction velocities can be detected even earlier, regardless of symptom onset.
CMTers with CMT1A will usually experience lower limb weakness, atrophy, and sensory loss before experiencing the same in the upper limbs. CMT1A disease severity has wide variability, and even within the same family.