Catalytic domain of the Serine/Threonine Kinase, Fungal Cyclin-Dependent protein Kinase Pho85STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Pho85 is a multifunctional CDK in yeast. It is regulated by 10 different cyclins (Pcls) and plays a role in G1 progression, cell polarity, phosphate and glycogen metabolism, gene expression, and in signaling changes in the environment. It is not essential for yeast viability and is the functional homolog of mammalian CDK5, which plays a role in central nervous system development. CDKs belong to a large family of STKs that are regulated by their cognate cyclins. Together, they are involved in the control of cell-cycle progression, transcription, and neuronal function. The Pho85 subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.