This is entirely in keeping with the more severe demyelinating CMT phenotype observed in our proband, compared to the natural history of CMT1A,24 and is akin to the phenotype expressed by patients carrying a PMP22 triplication.14 The relationship between gene dosage (and by extension protein concentration) and expressed phenotype is complex and largely influenced by the biological function of the protein of interest.4 In CMT1A, both PMP22 mRNA and protein have been shown to be elevated in dermal nerve, although there is substantial variation in these levels.20,25. The gene discussed is PMP22; the disease is Charcot-Marie-Tooth disease.