APOE and early-onset autosomal dominant Alzheimer disease: While APOE and ApoA1 are the most abundant apolipoproteins in the CSF and highly enriched in the brain (Borghini et al, 1995; Koch et al, 2001), APOC1—a less‐abundant brain apolipoprotein—was implicated in Alzheimer disease although its regulation and possible role is poorly understood (Petit‐Turcotte et al, 2001).