It is becoming apparent that an increase in the rate of protein degradation represents one mechanism for the regulation of key proteins during the cell cycle (1 ). The importance of protein degradation in the control of cell proliferation is elegantly illustrated by the regulated synthesis and destruction of the cyclin family of proteins during the life cycle of a cell. The cyclins are a group of proteins that influence the function of a series of intracellular kinases (the cyclin-dependent kinases or CDKs). The activation of specific CDKs is required during specific phases of the life cycle of a cell (this life cycle is known as the cell cycle). For example, the longest phase of the life cycle of a cell is the phase known as G1, or gap 1, which precedes the replication of cellular deoxyribonucleic acid (DNA) during S phase. Cells in G1 contain relatively high levels of two cyclins, known as cyclin D and cyclin E. These two cyclins are degraded as cells enter S phase and begin to replicate their DNA. The orderly synthesis and degradation of the cyclins is critical to proper progression of cells through the phases of the cell cycle.