RGS6 and Parkinson disease: The SNc thus has two major subsets of mDA neurons with differential vulnerability to Pitx3 knockout: TH+/Pitx3+/Rgs6+/Calb− neurons in vSNc (Pitx3-dependent, PD vulnerable) and TH+/Pitx3−/Rgs6−/Calb+ cells in dSNc (Pitx3-independent, PD resistant).