As recently vigorously debated, disease progression in PD could be explained by a prion-like propagation of α-synuclein itself across synapses (reviewed in: Walsh and Selkoe 2016; Rey et al. 2016); alternatively, an ‘initiating factor’, i.e., an environment-derived pathogen that leads to α-synuclein dysregulation within affected neurons (rather than misfolded α-synuclein species themselves) could gradually propagate across synapses. The gene discussed is SNCA; the disease is Parkinson disease.