Expression of ubiquitin genes under normal and stress conditions. Ubiquitin (Ub) is encoded by four different genes: Uba52, Uba80, Ubb, and Ubc. Two monoubiquitin genes (Uba52 and Uba80) and two polyubiquitin genes (Ubb and Ubc) are structured as shown. Under normal conditions, four ubiquitin genes are continuously transcribed, translated, and deubiquitinated to produce available free Ub. Under stress conditions, the expression of polyubiquitin genes is upregulated. As a result, cellular free Ub pool levels increase under stress conditions. For simplicity, only mouse ubiquitin genes are shown here.