Linkage specific, non-hydrolyzable di-ubiquitin is resistant to the activity of enzymes (DUB's) that cleave the isopeptide linkage between adjacent ubiquitin molecules. It can be used to investigate binding interactions between di-ubiquitin and proteins that contain elements such as ubiquitin-associated domains (UBAs) or ubiquitin-interacting motifs (UIMs). This product may also be useful in exploring the role of unanchored poly-ubiquitin chains in some signaling pathways.