Ubiquitination has been progressively recognized over the past quarter of a century or so as a post-translational regulatory mechanism operating in almost all the fundamental processes of cells. Mono-ubiquitination and poly-ubiquitination of proteins play a part in the regulation of transcription, translation, DNA repair, endocytosis, cell signaling, the cell cycle, and both lysosomal and proteasomal protein degradation. Yet fundamental questions about its mechanism remain unanswered. This series will explore what is currently known about the mechanisms of assembly of the diverse known ubiquitin signals, their disassembly and recognition, and their functions in cells. Relevant research papers published in BMC Biology will be added to the series, with commentary to explain their significance, or put them in context for nonspecialist readers, and submissions will be welcomed.