Inactive serine/threonine-protein kinase that plays a role in cell cycle progression and neuronal differentiation. Expressed in the brain, neurons and glial cells. Also expressed in highly differentiated cells, such as the serous acini in the parotid gland, distal and proximal tubules of the kidney, tubules of the seminal gland, Kupffer cells and some hepatocytes in the liver, and some cells in the germinal center of lymph nodes (at protein level). Down-regulated in primary brain tumors. The truncated protein kinase domain is predicted to be catalytically inactive; has lost the main activatory autophosphorylation site and the conserved key residues involved in phospho-substrate. The C-terminal region (containing the POLO box domain) is sufficient for inducing cell cycle arrest. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDC5/Polo subfamily.