For example, in DYT1 dystonia, a polymorphism on the non-mutated Tor1A allele (D216H) can reduce the penetrance of the disease-causing Tor1A three base pair deletion (c.907_909delGAG) to ∼ 3 % [4], [5]. The gene discussed is TOR1A; the disease is early-onset generalized limb-onset dystonia.