APOE and Alzheimer disease: ApoE2 has cysteine residues at positions 112 and 158; ApoE3 has one substitution (Cys158Arg), and ApoE4 has two (Cys112Arg, Cys158Arg).11 The worldwide prevalence of these variants is 8.4% (ApoE2), 77.9% (ApoE3), and 13.7% (ApoE4), respectively.11 The ApoE4 variant has been associated with health problems that include sporadic AD,12 worse outcomes after traumatic brain injury,13,14 and greater risks of kidney and cardiovascular disease.15,16 By contrast, ApoE2 seems to protect against cognitive decline.17–19