Dysregulated TrkB cell surface expression was also found in dSPNs and iSPNs in the striatum of Pitx3−/− mice, indicating that altered BDNF/TrkB signaling is not restricted to DRD1-expressing dSPNs, but also occurs in DRD2-expressing iSPNs and thus could contribute to the pathophysiology of specific disorders, such as PD, Huntington’s disease, and dystonia. Here, PITX3 is linked to Parkinson disease.