This fluorescent (FRET) peptide substrate contains the wild-type amyloid precursor protein (APP) β-secretase cleavage site. Mca-SEVKMDAEFRK(Dnp)RR- amide has been used for assaying β-secretase-like activity of thimet oligopeptidase (TOP, EC 3.4.24.15). The results suggested that TOP is a potential β-secretase candidate and is involved in the processing of APP in vivo.