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Nature Cell Biology 4, E121–E123 (1 May 2002) | doi:10.1038/ncb0502-e121

Ubiquitin chained and crosslinked

Daniel Finley

The proteasome is the principal ATP-dependent protease in eukaryotic cells, and as the sole known mediator of ubiquitin–protein conjugate breakdown, it is fundamental to the regulation of a variety of biological processes (Fig. 1).