APOE and Parkinson disease: It is likely that different forms of PD progression are influenced by different genetic variants and processes, although there may be some shared factors such as APOE. Furthermore, the heterogeneity of the cohorts (e.g. disease stage, inclusion criteria), in addition to the difficulty in measuring progression through clinical scales, may explain why we have not replicated previous findings even for the same outcome measures as previous studies, namely H&Y3+.