MAPT and Parkinson disease: The prion protein was the first protein to be detected in exosomes of ovine CSF infected by prion disease (Vella et al., 2008a), followed by phosphorylated tau (Saman et al., 2012) and amyloid β in models of AD (Yuyama et al., 2015) and α-synuclein in patients with PD (Shi et al., 2014; Yang et al., 2015).