TOR1A and Dystonia: Even in a relatively “simple” case such as DYT1-dystonia, there remain many unanswered questions with respect to genotype–phenotype correlations, for example, (i) what determines penetrance of the DYT1-mutation, i.e., why do some mutation carriers develop the clinically manifest disorder, while many others do not and (ii) can mutations other than the classic GAG deletion cause dystonia?