It is useful to artificially control the expression levels of a protein of 
interest (POI), not only for its characterization in vivo, but also for the 
modulation of biological pathways. Overexpression of a POI is relatively easy 
because it is possible to drive its expression from a transgene encoding the POI 
under the control of a strong promoter. However, it is more challenging to 
reduce or deplete the expression of a POI. A protein domain called "degron", 
which induces rapid proteolysis by the proteasome, can be used for this purpose. 
Degron-based technologies for the conditional depletion of POI--degron fusion 
proteins have been developed by exploiting various pathways leading to 
proteasomal degradation. Compared with other depletion technologies that control 
the expression levels of POIs at the DNA or mRNA levels, these protein-depletion 
approaches are advantageous in terms of specificity, reversibility, and the time 
required for depletion. Current conditional degron-based technologies are 
described and discussed.