Hydrophobic deletions in the amyloid core destabilises the fibril. We formed fibrils from all PI3K-SH3 variants through seeding of monomeric samples with WT fibrils, in order to ensure a structural coherence between all samples, and exposed them to increasing concentrations of urea. Once the samples were fully equilibrated, we measured the soluble protein fraction from intrinsic fluorescence emission intensity ratios at 340 nm/310 nm or 350 nm/330 nm. We have demonstrated that different measurement approaches used here are comparable 15 (Supporting Results), globally fitted the full dataset (Figure ) using the cooperative polymerization model and calculated the change in thermodynamic stability, ∆∆G A-U (Table ).