APOE and Alzheimer disease: Some studies have defined AD subtypes by age of onset (e.g. early onset vs late onset), genetics (e.g. apoE2 vs apoE3 vs apoE4 or familial AD vs sporadic AD), by neuropathology phenotype (e.g. limbic predominant vs hippocampal sparing vs typical), by rate of progression (e.g. rapidly progressive AD vs typical AD), or more recently using unbiased ‘omics approaches.