Although not considered in detail in our present analysis, the fluorescence signal of the fibrils during FIDA provides valuable qualitative information about their properties. Fibrils that are prone to selfassociation form larger aggregates that are detected as signal spikes whose intensity is related to their size. Conversely, isolated, and homogenized (sonicated) fibrils are often small enough to appear as continuous signal of non-diffusive particles (Figure ). Such information can be useful in attempts to optimize solution conditions towards favouring well-defined individual fibrils, something that is a requirement for structural analyses of amyloid fibrils, e.g. by AFM or cryoEM. Requires presence of tryptophan residues and measurable difference of their fluorescence spectra in the monomeric versus fibrillar state.