Clinical variability in two pairs of identical twins with the Charcot-Marie-Tooth disease type 1A duplication.

We report two pairs of male homozygotic twins in two unrelated families with the Charcot-Marie-Tooth disease type 1A duplication. Homozygosity was supported by DNA analysis. There was remarkable congruity of conduction velocities between the left and right side of each twin and between twin brothers. The similarity and symmetry of the electrophysiologic deficit contrast with the variable and asymmetric clinical presentations. Variability of clinical expression in these patients with identical mutations suggests the action of stochastic factors or environmental modulation of disease severity.