β-Secretase (BACE1) is an important enzyme in Alzheimer's disease (AD).45 β-amyloid (Aβ) peptides are generated by sequential cleavages from BACE1, and the accumulation of Aβ is responsible for the pathogenesis of AD.46 Recently, Tian and colleagues have created a FRET-based TP ratiometric fluorescent probe AF633mCyd for the bioimaging and sensing of BACE1 in different regions of AD mouse brain. Here, BACE1 is linked to early-onset autosomal dominant Alzheimer disease.