Activation of the mitotic cyclin:Cdc2 complexes at mitosis requires the removal of the inhibitory phosphate groups on Cdc2. This dephosphorylation is achieved by the activity of the Cdc25 family of phosphatases. The Cdc25 members, Cdc25A, Cdc25B, and Cdc25C are kept inactive during interphase and are activated at the G2/M transition. Cyclin B1:Cdc2 itself appears to participate in the full activation of Cdc25 in a process that involves an amplication loop (see Wolfe and Gould, 2004). The initial activation of the cyclin B1-Cdc2 complex occurs in the cytoplasm in prophase (Jackman et al., 2003). Cdc25B, which is present at highest concentrations in the cytoplasm at this time, is thought to trigger the activation of cyclin B1-Cdc2 (Lindqvist et al. 2004; Honda et al., 1993). Active cyclin B:Cdc2 then phosphorylates and activates Cdc25C and stabilizes Cdc25A (Strausfeld et al., 1994; Hoffman et al.,1993; Mailand et al, 2002). This creates positive feedback loops that allows Cdc25A and Cdc25C to dephosphorylate and further activate Cdc2.