PMP22 and Charcot-Marie-Tooth disease type 1A: In the most common form of CMT, CMT type 1 A (CMT1A), skin biopsy was able to disclose both macroscopic nerve abnormalities (e.g., shortened internodal length) and abnormalities at the molecular level, such as increased levels of the PMP22 protein, consistent with the etiological duplication of the relative gene in CMT1A.