The null model demonstrates behavior similar to fibril growth without any shaving phases (Fig. A and Fig. ). Both models A and B predict increased rates of mass addition only for very short periods of time, which is not what is observed. Rather the accelerated growth rates are maintained for many hours. Accelerated mass association to the sensor surface for times extending beyond tens of minutes are only expected in kinetic models, where the formation of new fibril ends occurs (Model C and D). Finally, only model D captures the immediate acceleration in fibril growth rates, which is observed in our experiments, suggesting that the additional fibril ends are made available during the Proteinase K degradation phase.