MAPT and Alzheimer disease: Given that, in AD and PD, proteins such as Aβ, tau or α-synuclein appear to ‘seed’ the accumulation of other neurotoxic proteins and promote neurodegeneration (Clavaguera et al., 2009; Langer et al., 2011; Lasagna-Reeves et al., 2012; Rey et al., 2013), we sought to determine whether a similar mechanism operates in SCA1.