Amyloid fibrils share a structural motif consisting of highly ordered β-sheets aligned perpendicular to the fibril axis ( 1, 2) . At each fibril end, β-sheets provide a template for recruiting and converting monomers ( 3) . Various amyloid fibrils often occur in the same individual, yet whether distinct protein aggregates aid or inhibit the assembly of heterologous proteins is unclear. In prion disease, different amyloid-like prion aggregate structures, or strains, are thought to be the basis of disparate disease phenotypes in the same species expressing identical prion protein sequences ( 4-7) . Here we focus on the interactions reported to occur when two pre-existing amyloids or two distinct prion strains occur together in the central nervous system.