These concentrations investigate only the linear growth regime, and as such any calculated Φ-values will only apply to this particular regime. The saturated regime is experimentally unavailable with the system presented here due to the fast growth rate of PI3K-SH3 fibrils in the investigated conditions. QCM measurements of fibril elongation are only sensitive to a finite growth window as the fibril ends eventually extend beyond the sensor sensitivity range. PI3K-SH3 fibril elongation enters the saturated regime at concentrations above 200 µM, at which consecutive measurements of relative elongation rates become unreliable (Figure ).