The ubiquitin-proteasome system degrades an enormous variety of proteins that 
contain specific degradation signals, or 'degrons'. Besides the degradation of 
regulatory proteins, almost every protein suffers from sporadic biosynthetic 
errors or misfolding. Such aberrant proteins can be recognized and rapidly 
degraded by cells. Structural and functional data on a handful of degrons allow 
several generalizations regarding their mechanism of action. We focus on 
different strategies of degron recognition by the ubiquitin system, and contrast 
regulatory degrons that are subject to signalling-dependent modification with 
those that are controlled by protein folding or assembly, as frequently occurs 
during protein quality control.