At the entry to mitosis, Cdc25 has a basal phosphatase activity that can partially activate Cdc2 by removing the inhibitory phosphates added by Wee1 and Mik1. Cdc2 can then feed back and phosphorylate Cdc25, further activating it. This process is linear and involves an intermediary kinase in addition to Cdc2. The amplification of Cdc2 activity needed for mitotic entry does not occur until Plk becomes activated. Cdc25 is then further activated, which results in the burst of Cdc2 activation needed for mitotic entry. Plk can directly activate Cdc25 and thus probably plays a role in the positive feedback loop that operates during p34cdc2 activation at the G2-M transition.