Resumen: The 99-amino-acid-long APP-carboxy-terminal fragment, named C99, is a membrane-bound peptide generated from the amyloid precursor protein (APP) by ß-secretase cleavage and is the direct precursor of amyloid beta (Aß). Here we describe a method for the quantification of C99. The amount of C99 is an indicative value of the amyloid pathology in an Alzheimer's disease (AD) model, and could be used as a marker to study AD progression in comprehensive experiments, including screening for new compounds and repurposing of drugs to treat AD.