SUMO (small ubiquitin-like modifier) emerged from the shadow of the well-established ubiquitin some 15 years ago when it was shown that a distinct conjugation pathway was responsible for SUMO modification. Since then it has been established that SUMO modifies over a thousand substrates and plays diverse roles in many important biological processes. Recognition of SUMO is mediated by short peptide sequences known as SIMs (SUMO-interaction motifs) that allow effector proteins to engage SUMO-modified substrates. Like ubiquitin, SUMO can form polymeric chains, and these chains can be recognized by proteins containing multiple SIMs. One protein that contains such a sequence of SIMs also contains a RING (really interesting new gene) domain that is the hallmark of a ubiquitin E3 ligase. This ubiquitin ligase known as RNF4 (RING finger protein 4) has the unique property that it can recognize SUMO-modified proteins and target them for ubiquitin-mediated proteolysis. Structural and biochemical analyses of RNF4 has shed light on the long sought after mechanism of ubiquitin transfer and illustrates how its RING domain primes the ubiquitin-loaded E2 for catalysis.