DDC and Parkinson disease: Phase I and phase II clinical trials of gene therapy for PD investigated or completed in the United States included AAV2-GAD (LeWitt et al. 2011; Niethammer et al. 2017; Kaplitt et al. 2007), AAV2-AADC (Christine et al. 2019, 2009; Mittermeyer et al. 2012; Muramatsu et al. 2010), and AAV2-NRTN (Bartus et al. 2013; Marks et al. 2016, 2010).