The documented inhibition of prolyl oligopeptidase by knottin-like cyclotides [13,130] should trigger scientific investigation of the potential of knottin-like peptides, to inhibit the enzymes involved in a cascade of reactions leading to the extracellular plaques of beta-amyloid peptide (Aβ) deposits in the brain observed in Alzheimer’s disease [131]. The gene discussed is PREP; the disease is Alzheimer disease.