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. Nuclear cyclin B1 is a substrate of SCFNIPA. Inactivation of NIPA by RNAi results in nuclear accumulation of cyclin B1 in interphase, activation of cyclin B1-Cdk1 kinase activity, and premature mitotic entry. Thus, SCFNIPA-based ubiquitination may regulate S-phase completion and mitotic entry in the mammalian cell cycle.