The measurement of aromatic L-amino acid decarboxylase activity using [18F]FDOPA is regarded as the gold standard to examine DA function and disease severity in Parkinson’s disease (Morrish et al., 1996; Punal-Rioboo et al., 2009), also shown in PD non-human primate model (Muramatsu et al., 2009; Emborg et al., 2013; Hallett et al., 2015) and clinical reports (Lindvall et al., 1990; Peschanski et al., 1994; Piccini et al., 2000, 2005; Ma et al., 2010) (refer to citation in Table 2). The gene discussed is DDC; the disease is Parkinson disease.