Regulates the G1/S phase transition of the cell cycle by binding and activating CDK1 and CDK2 (PubMed:15611625). Contributes to CDK2 activation without promoting CDK2 phosphorylation, by inducing a conformation change of the CDK2 T-loop that obstructs the substrate-binding cleft prior to kinase activation. Interferes with CDKN1B-mediated inhibition of CDK2. Mediates cell survival during the DNA damage process through activation of CDK2 (By similarity). Interacts with CDK1, CDK2 and CDKN1B/KIP1 (PubMed:15611625). Identified in a complex with CDK2 and CDKN1B/KIP1, where it interacts primarily with CDK2 (By similarity). Expressed at a high level in testis. Also expressed in the adult ovary and in immature oocytes. The C-terminus is required for CDK2-activation, but not CDK2-binding. Belongs to the Speedy/Ringo family.