APP and early-onset autosomal dominant Alzheimer disease: As a control, DAPT also blocked secretion of the Alzheimer's disease‐linked Aβ peptide from HEK293 cells transfected with C99 (Fig 1C), a C‐terminal fragment of the amyloid precursor protein (APP) that is an established substrate for γ‐secretase (Lichtenthaler et al, 1999).