ubiquitin

noun

ubiq·​ui·​tin yü-ˈbik-wət-ən 

: a chiefly eukaryotic protein that when covalently bound to other cellular proteins marks them for proteolytic degradation especially by a proteasome

What they found was a protein called ubiquitin, so named because it is ubiquitous, occurring in every cell of the body. Through a series of clever studies, they found that cells marked proteins destined for destruction by attaching one or more ubiquitin molecules to them.

—Thomas H. Maugh II, The Los Angeles Times

Ubiquitin tagging directs the movement of important proteins in the cell, determining, for example, whether they end up on the cell membrane or in an internal vacuole, where they are destroyed without the proteasome's help.

—Jean Marx, Science