The regulated oscillation of protein expression is an essential mechanism of cell cycle control. The SCF class of E3 ubiquitin ligases is involved in this process by targeting cell cycle regulatory proteins for degradation by the proteasome, with the F-box subunit of the SCF specifically recruiting a given substrate to the SCF core. NIPA (nuclear interaction partner of ALK) is a human F-box-containing protein that defines an SCF-type E3 ligase (SCFNIPA) controlling mitotic entry. Assembly of this SCF complex is regulated by cell-cycle-dependent phosphorylation of NIPA, which restricts substrate ubiquitination activity to interphase.