Bora (Protein aurora borealis) is a key activator of Aurora Related Protein Kinase A (ARK-1), which is a centrosome-associated serine/threonine kinase that regulates centrosome maturation, bipolar spindle assembly and chromosome segregation during mitosis. Bora is localized to the nucleus until mitosis is initiated, then translocates to the cytoplasm in a Cdc2 dependent manner. Activation of Cdc2 initiates the release of Bora into the cytoplasm where it can bind and activate ARK-1. PLK1 (polo-like kinase-1) interacts with Bora to control the accessibility of its activation loop for phosphorylation and activation by ARK-1. Bora and ARK-1 cooperatively activate PLK1 and control mitotic entry. Bora mutants result in multipolar spindles in mitosis identical to those observed when ARK-1 function is blocked.  Thus, the ARK1-Bora-PLK1 regulatory circuit in mammalian cells elucidates a key mechanism in cell cycle regulation. At least three isoforms of Bora are known to exist.