APOE and Parkinson disease: Apolipoprotein E isoforms are associated with Aβ plaque deposition (Irwin et al., 2013; Paul et al., 2016) and α-synuclein aggregation (Emamzadeh et al., 2016; Emamzadeh, 2017) in PD, with ε4 considered to carry a higher risk than either of the others (ε2 and ε3).