SNCA and Parkinson disease: Despite this, Breid’s study had a very small sample size, used an α-synuclein over-expressing model of PD which doesn’t accurately capture the genotype of most PD patients (only 5–10% of PD patients have mutations in genes like SCNA, which encodes α-synuclein) [2], and was subject to a confirmation bias because α-synuclein spread is more likely to be observed in α-synuclein over-expressing mice.