PMP22 and Charcot-Marie-Tooth disease: These simple considerations lead to the conclusion that under heterozygous T118M/WT conditions, T118M PMP22 is incompletely penetrant as a cause of CMT and related neuropathies: its incidence in the US population is 1:120, but only 1:2500 in this population are known to be afflicted with any form of CMT.