APLP1 and Alzheimer disease: Aβ42 to Aβ40 ratio was estimated in PSEN1 mutant AD using surrogate amyloid precursor-like protein-1-derived Aβ-like peptide (APL1β), including APL1β28. Relatively high ratio of CSF Aβ42 surrogate in PSEN1 mutant AD without an increase of Aβ42 secretion in the brain.