Ubiqutination (or ubiquitylation) is a post-translational modification of proteins. Ubiquitin is a small protein that is conjugated to a lysine residue of the target protein in a multi-step process involving multiple ubiquitin conjugating enzymes (also called ubiquitin ligases). Poly-ubiquitination serves as a signal for degradation of the target protein by the 26S proteasome. Mono-ubiquitination is a regulatory modification that alters the function of many nuclear proteins. Ubiquitin is removed by ubiquitin deconjugating enzymes, also called de-ubiquitinases. Active Motif offers a number of recombinant proteins involved in ubiquitin pathways.