This article is part of a Minifocus on Ubiquitin. For further reading, please see related articles: 'Ubiquitin and SUMO in DNA repair at a glance' by Helle D. Ulrich (J. Cell Sci. 125, 249-254), 'Emerging regulatory mechanisms in ubiquitin-dependent cell cycle control' by Annamaria Mocciaro and Michael Rape (J. Cell Sci. 125, 255-263), 'The role of ubiquitylation in receptor endocytosis and endosomal sorting' by Kaisa Haglund and Ivan Dikic (J. Cell Sci. 125, 265-275), 'Cellular functions of the DUBs' by Michael J. Clague et al. (J. Cell Sci. 125, 277-286), 'Non-canonical ubiquitin-based signals for proteasomal degradation' by Yelena Kravtsova-Ivantsiv and Aaron Ciechanover (J. Cell Sci. 125, 539-548) and 'No one can whistle a symphony alone how different ubiquitin linkages cooperate to orchestrate NF-kappa B activity' by Anna C. Schmukle and Henning Walczak (J. Cell Sci. 125, 549-559).