APOE and Parkinson disease: Gene susceptibility has been investigated in PD with a number of early human epidemiological studies and meta-analyses investigating the role of Apolipoprotein E (ApoE) and cholesterol metabolism in PD pathogenesis [184,185,186], following the discovery that carriers of the ApoE-ε4 isoform gene showed a higher probability of progression onto PD, while those with ApoE-ε2 isoform showed a level of protection against developing PD [184,187,188].