APOE and Parkinson disease: Further characterization of the two clusters of PD, cluster 2 and cluster 3, reveals that the PD subjects in cluster 3, characterized by a highly sparse brain DAT network, had lower DAT levels (severe DAT deficit, as quantified by DAT SBr) compared to PD subjects in cluster 2 in almost all six striatal regions assessed across APOE genotypes (see Supplementary Figure S2).