Temporal control of ubiquitin—proteasome mediated protein degradation is critical for normal G₁ and S phase progression. Recent work has shown that central to the temporal control mechanism is a relationship between newly identified E3 ubiquitin protein ligases, designated SCFs (Skp1—cullin—F-box protein ligase complexes), which confer substrate specificity on ubiquitination reactions and the activities of protein kinases that phosphorylate substrates destined for destruction at specific sites, thereby converting them into preferred targets for ubiquitin modification catalyzed by SCFs. The constituents of SCFs are members of evolutionary conserved protein families. SCF-based ubiquitination pathways may play a key role in diverse biological processes, such as cell proliferation, differentiation and development.