Promotes progression through the cell cycle via binding and activation of CDK1 and CDK2. Involved in the spindle-assembly checkpoint. Required for recruitment of MAD2L1, BUBR1 and BUB1 to kinetochores. Required for the correct localization of the active form of Aurora B in prometaphase. Interacts with CDK1 and CDK2. Interacts with AURKB. Colocalizes with tubulin gamma during interphase. During mitosis, enriched at the spindle poles and the midbody during telophase. Expressed in a variety of tissues including bone marrow, kidney, small intestine, liver, placenta and testis. The C-terminus is required for CDK2-activation, but not CDK2-binding. Belongs to the Speedy/Ringo family.