Regulates the G1/S phase transition of the cell cycle by binding and activating CDK1 and CDK2 (PubMed:12972555). 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 (PubMed:28666995). Mediates cell survival during the DNA damage process through activation of CDK2 (PubMed:12839962). Interacts with CDK1 (By similarity). Interacts with CDK2 (PubMed:11980914, PubMed:12839962, PubMed:12972555, PubMed:28666995). May interact with CDKN1B/KIP1 (PubMed:12972555). Identified in a complex with CDK2 and CDKN1B/KIP1, where it interacts primarily with CDK2 (PubMed:12972555, PubMed:28666995). Highly expressed in testis. Expressed at a low level in wide range of tissues including bone marrow, brain, heart, kidney, colon, liver, placenta, spleen, skeletal muscle, salivary gland, thyroid gland, thymus, trachea and uterus. Expressed at a slightly higher level in adrenal gland, cerebellum, small intestine, lung, prostate and trachea. Expression is cell cycle-dependent, being restricted to cells in G1/S phase. The C-terminus is required for CDK2-activation, but not CDK2-binding. Belongs to the Speedy/Ringo family.