The phosphorylation and dephosphorylation of proteins on serine and threonine residues is an essential means of regulating a broad range of cellular functions in eukaryotes, including cell division, homeostasis and apoptosis. A group of proteins that are intimately involved in this process are the serine/threonine (Ser/Thr) protein kinases. STK33 (serine/threonine kinase 33) is a 514 amino acid protein that belongs to the CaMK (calcium/calmodulin dependent kinase) subfamily of structurally related serine/threonine kinases. Widely expressed at low levels with predominant expression in testis, lung, retina and fetal organs such as brain, heart and spinal cord, STK33 contains one protein kinase domain and functions as a Ser/Thr protein kinase with a possible role in spermatogenesis. The gene encoding STK33 lies within a region on chromosome 11 that has been associated with a variety of defects, including Long QT syndrome, T-cell leukemia, Beckwith-Wiedemann syndrome, Usher syndrome 1C and various other malignancies.