A 26-residue fragment of the Aβ protein; formed via post-translational processing of APP by α- and γ-secretases; has been found in postmortem cerebral cortex from patients with Alzheimer's disease that have diffuse amyloid plaques and in cerebellar preamyloid lesions in patients with Down's syndrome; forms amorphous aggregates in vitro that bind less thioflavin T than Aβ (1-40) and Aβ (1-42); is cytotoxic to SH-SY5Y and IMR-32 cells in a concentration-dependent manner and induces cleavage of caspase-3 and PARP in SH-SY5Y cells at 20 μM