Ubiquitin is a small protein with a big impact. From yeast fungi to humans, it serves as a molecular tag that regulates many cellular processes. Ubiquitin ligases are indispensable labeling machines in this tagging process: They attach ubiquitin to target proteins. If this tagging fails, processes in the cell can be pathologically altered. A team led by Sonja Lorenz at the Max Planck Institute (MPI) for Multidisciplinary Sciences has now visualized the ubiquitin ligase HACE1 bound to an important target protein in 3D. The researchers were thus able to elucidate how HACE1 recognizes proteins and how this process is regulated.