APP and Alzheimer disease: HPLC-purified 125I-Aβ40, 125I-Aβ40 H13G, 125I-Aβ40 H13R, and 125I-Aβ40 H13,14G were incubated in vitro with unfixed cryosections of brain from a 20-month old APP, PS1 AD transgenic mouse.