Subjects
We examined a total of 251 members from 4 kindreds with ET. The mean (±SD) age at onset of ET varied among the 4 kindreds between 19.0±11.4 years and 45.6±7.4 years. Three of the kindreds had a total of 41 members with the combination of ET and dystonia, typically manifested as torticollis or dystonic writers' cramp. In 1 of the kindreds, ET seemed to be associated with malignant hyperthermia. One kindred represerted "pure" ET without any associated disorders.

Methods
In addition to detailed clinical assessments, we conducted computer simulations on the families' pedigrees using a model that presumed an autosomal dominant inheritance pattern with high penetrance.

Results
Although there was evidence of clinical heterogeneity between the families, the duration of symptoms directly correlated with the severity of disease. The computer simulations indicated that 3 of the 4 pedigrees had enough power to generate a significant linkage result in a total genome search with highly polymorphic markers.

Conclusions
This study confirms the frequent coexistence of ET and dystonia in individual families. Computer simulations can be used to determine the power of the family to detect a linked marker. Identification of the defective gene(s) will enable a better understanding and classification of these common movement disorders.