Before revision:  stochasticity is modeled at the biological function level under the assumption that even if the expression levels of the input nodes of an update function guarantee activation or degradation there is a probability that the process will not occur due to stochasticity in the process, for instance, some of the chemical reactions encoded by the function fail to occur which lead to a function failure .
After revision:  stochasticity is modeled at the biological function level under the assumption that even if the expression levels of the input nodes of an update rule guarantee activation or degradation there is a probability that the process will not occur due to stochasticity in the process, for instance, some of the chemical reactions encoded by the function fail to occur which lead to a function failure .