The split-ubiquitin technique can monitor alterations in the conformation of proteins and can be used to detect the interaction between two proteins in living cells. The technique is based on unique features of ubiquitin, the enzymes of the ubiquitin pathway, and the reconstitution of a native-like ubiquitin from its N- and C-terminal fragments. By exploiting the reassociation of a protein from its defined fragments to monitor protein interactions, the split-ubiquitin assay served as the prototype of a still growing number of split-protein sensors to analyze protein function within living cells.