Many studies focus on the prodromal stages of AD—typically patients diagnosed with subjective or mild cognitive impairment (SCI/MCI), genetically at risk (e.g., carriers of the apolipoprotein E ε 4 allele), and/or those who are positive for an AD biomarker, e.g., CSF analysis or amyloid-ligand PET. This evidence concerns the gene APOE and Alzheimer disease.