Ubiquitin is a tiny protein containing only 76 amino acids, yet it participates in an astonishing range of cellular activities. It helps determine which proteins are destroyed, which are moved, which signaling pathways are activated, how damaged DNA is repaired, whether inflammation begins or ends, when a cell divides, whether a damaged mitochondrion is eliminated, and sometimes whether the cell itself survives or dies. How can the same tiny molecule mean so many different th