SNCA and Parkinson disease: APP (amyloid beta precursor protein) and SNCA (alpha-synuclein), which are implicated, respectively, in Alzheimer’s (Rovelet-Lecrux et al., 2006) and Parkinson’s diseases (Singleton et al., 2003), are a case in point (Gimelbrant et al., 2007; Eckersley-Maslin et al., 2014; Gendrel et al., 2014).