One of the most informative methods to study the roles of individual proteins in cell functions is based on changing their intracellular concentrations. Genetic knockouts or knockdowns are most commonly used for the purpose. However, acting directly at the level of an expressed protein is more informative or convenient to perform in some cases. This action should ideally be controlled in time and reversible. The review analyzes the current data on systems developed to achieve controlled degradation of proteins via their ubiquitination with subsequent proteasome-mediated degradation or other mechanisms.