APP and Alzheimer disease: Monomers and fibrils of Aβ are not very toxic to neurons, but the soluble oligomers are neurotoxic in culture leading to the hypothesis that they are responsible for the neurodegeneration in AD.[45] The major support for a role of Aβ42 in AD came from the discovery of a small number of families with familial AD (FAD) linked to the AβPP gene.