In collaboration with Katrin Rittinger at the MRC-NIMR in London, the Dikic group solved another molecular puzzle about the formation of specific Ubiquitin chains. In the current issue of Nature, they report the crystal structure of the catalytic core of HOIP, the critical enzymes involved in forming linear (Met1-linked) Ubiquitin chains. These chains are important regulators of cellular signalling, and knowing the molecular structure of the complexes involved is an important step forward to understanding how these pathways control innate immunity and inflammation.