1) A highly conserved 76-amino acid peptide universally found in eukaryotic cells that functions as a marker for intracellular protein transport and degradation. Ubiquitin becomes activated through a series of complicated steps and forms an isopeptide bond to lysine residues of specific proteins within the cell. These "ubiquitinated" proteins can be recognized and degraded by proteosomes or be transported to specific compartments within the cell.

2) Ubiquitous intracellular protein which, when covalently attached to other proteins, marks them for degradation or targeted transport.

3) A family of proteins found in all cells of higher organisms and one whose structure has changed minimally during evolutionary history. It is involved in at least two processes; histone modification and intracellular protein breakdown. Ubiquitin-dependent proteolysis by the 26S proteasome plays a pivotal role in cell cycle progression as well as in tumorigenesis. It is encoded by at least three genes; UBB, UBC and UBD.