This domain family is found in eukaryotes, and is approximately 140 amino acids in length. The family is found in association with pfam00069. Polo-like kinase 1 (Plx1) is essential during mitosis for the activation of Cdc25C, for spindle assembly, and for cyclin B degradation. This family is Polo kinase kinase (PKK) which phosphorylates Polo kinase and Polo-like kinase to activate them. PKK is a serine/threonine kinase.