One model proposes that loss of the GTP-cap causes the GDP-containing protofilaments to shrink. Based on this GTP-cap model, catastrophe happens randomly. The model proposes that an increase in microtubule growth will correlate with a decrease in random catastrophe frequency or vice versa. The discovery of microtubule-associated proteins that change the rate of catastrophe while not impacting the rate of microtubule growth challenges this model of stochastic growth and shrinkage.