The Aβ4–15/Aβ1–15 ratios in the insoluble pool indicated an increased accumulation of truncated Aβ with Phe-4 (phenylalanine residue) N-terminus in Alzheimer’s disease, reaching almost equal concentration as the BACE1 cleaved Aβ N-terminus (Asp-1), making it the next most abundant N-termini present in Alzheimer’s disease brains. Here, BACE1 is linked to early-onset autosomal dominant Alzheimer disease.