The N-end rule pathway is a proteolytic system in which recognition components 
(N-recognins) recognize a set of N-terminal residues as part of degradation 
signals (N-degrons). Two studies in this issue report the structures of Ubr 
boxes, a substrate recognition domain of eukaryotic N-recognins. We discuss how 
eukaryotic and prokaryotic N-recognins use a similar molecular principle to 
recognize a different set of N-degrons.