SNCA and multiple system atrophy: To test the hypothesis that heterogeneity in α-synuclein seeding exists in MSA, we selected six from the 15 MSA subjects (Fig. 4h), and classified them as low seeders (subjects MSA 1 and MSA 6), intermediate seeders (subjects MSA 3 and MSA 4) and high seeders (subjects MSA 2 and MSA 5) according to their α-synuclein seeding behavior.