Speedy (Spy1) is a cell cycle regulatory protein which activates CDK2, the major kinase that allows progression through G1/S phase and further replication events [1]. Spy1 expression overcomes a p27-induced cell cycle arrest to allow for DNA synthesis, so cell cycle progression occurs due to an interaction between Spy1 and p27 [1]. Spy1 is also known as Ringo protein A.

1: Spy1 enhances phosphorylation and degradation of the cell cycle inhibitor p27. McAndrew CW, Gastwirt RF, Meyer AN, Porter LA, Donoghue DJ; Cell Cycle. 2007;6:1937-1945. PMID:17671428