Ubiquitin is a small protein that is found in almost all cellular tissues in humans and other eukaryotic organisms, which helps to regulate the processes of other proteins in the body. Ubiquitin was discovered in early 1975 by Gideon Goldstein and further characterized throughout the 1970s to 1980s. UBB, UBC, UBA52 and RPS27A genes of the human genome code for ubiquitin. Ubiquitin is a small protein that can be covalently linked to lysine residues of proteins targeted for intracellular degradation by proteasomes. The ubiquitin (Ub) system plays a pivotal role in protein homeostasis by regulating the turnover of proteins important in a plethora of regulatory pathways such as DNA damage and repair, cell cycle progression, apoptosis, receptor-mediated endocytosis, and signal transduction. Alterations in this pathway often lead to pathological conditions for the host. The Ub pathway involves an unusual combination of many specific enzymatic proteins that target nearly all short-lived and abnormal proteins for proteasomal degradation. The half-life of each protein is precisely regulated and the degradation is extremely selective and controlled at the level of ubiquitination. The role of Ub as a proteolytic signal and the requirement of such highly organized machinery for the formation of an isopeptide bond between Ub and a substrate have been well characterized. 

The Ub–Ub isopeptide bond is a major contributor in the sorting of substrates for either degradation or some other physiological function. Deubiquitinating enzymes (DUBs) further add complexity to this scheme by acting as proof-reading enzymes and thus increasing the specificity of this system. The existence of distinct Ub chains, differing only in the number of Ub–Ub linkages, not only complicates the process but also reveals the potential of alternative molecular mechanisms taking place as a result of ubiquitination, other than degradation.