Ubiquitin is a small protein found in all eukaryotic cells. It plays a crucial role in regulating various cellular processes by tagging other proteins for degradation. This tagging process is essential for maintaining cellular health, as it helps remove damaged or unneeded proteins. The attachment of ubiquitin to a target protein is known as ubiquitination. This process involves a series of enzymes that work together to add ubiquitin molecules to the protein, marking it for destruction by the proteasome, a cellular structure responsible for breaking down proteins.