Taking into account that in the blood plasma and senile plaques of AD patients, in most cases, a mixture of monomeric and aggregated forms of Abeta peptides is detected [62], the study of the structure, properties, and toxicity of Aβ40 and Aβ42 fibrils formed as a result of seeding and cross-seeding is of interest. The gene discussed is APP; the disease is Alzheimer disease.