γ-secretase cleaves multiple substrates with essential roles from development to neurodegeneration, and its aberrant processing underlies human disorders including Alzheimer's disease (AD). Tian et al. now identify a domain in the β-amyloid precursor protein (APP) that inhibits γ-secretase activity and show that certain familial AD–linked APP mutations within this domain impair this inhibition, resulting in increased β-amyloid generation. The study thus reveals a novel mechanism whereby γ-secretase's activity is influenced by its own substrate.