TH and dystonia 5: Consistent with its essential role in dopamine homeostasis, homozygous missense mutations in TH have been associated with dopamine-related phenotypes, such as Segawa's syndrome, L-DOPA responsive infantile parkinsonism, and L-DOPA responsive dystonia (DRD) (Furukawa et al., 2001; Hertz et al., 2006).