Different forms of ubiquitin and ubiquitin-modified proteins. Ubiquitylation may result in addition of a single ubiquitin moiety or a branched multi-ubiquitin chain to the target protein lysine(s). Note that a ubiquitin molecule possesses 7 inner lysine residues that can serve as attachment sites of the next ubiquitin moiety, resulting in the formation of the chains that have a different structure and topology. Functionally significant amino acids are marked as follows: Kn and Kn' – lysine residue(s) that can serve as attachment sites of the ubiquitin moiety; G76 – ubiquitin C-terminal glycine residue participating in the isopeptide bond formation. Poly-ubiquitin refers to a ubiquitin fusion protein, a precursor form polymerized "head-to-tail".