Cerebrospinal fluid (CSF) biomarkers for Alzheimer’s disease (AD) reflect brain biochemistry. Using combined immunoprecipitation and mass spectrometry, we have shown that amyloid beta 1-15 (Abeta1-15) is produced by concerted beta- and alpha-secretase cleavage of amyloid precursor protein (APP) and that the relative levels of Abeta1-16 in AD compared to controls are increased. Furthermore, drug-induced gamma-secretase