"Oscillatory reactions of the epigenetic type are also well known. They occur as a consequence of regulatory processes at the cellular level. Proteins are generally stable molecules, whereas catalysis is a very fast process. Thus, it is not unusual for the protein level in a cell to be too high, in which case other bodily substances act to suppress the synthesis of macromolecules. Such feedback gives rise to oscillations and has been studied in detail in, for example, the regulation of the lactose operon in the bacterium Escherichia coli."