Serine / threonine-protein kinase 1, also known as Lymphocyte-oriented kinase, STK1 and LOK, belongs to the protein kinase superfamily, STE Ser / Thr protein kinase family and STE2 subfamily. Protein kinases constitute a large superfamily of enzymes with key regulatory functions in nearly all signal transmission processes of eukaryotic cells. The Ste2 family of serine/threonine kinases plays an important role in numerous cellular functions such as growth, apoptosis, and morphogenesis. STK1 is similar to several known polo-like kinase kinases. It can associate with and phosphorylate polo-like kinase 1, and overexpression of a kinase-dead version of the protein interferes with normal cell cycle progression. STK1 can also negatively regulate interleukin 2 expression in T-cells via the mitogen activated protein kinase kinase 1 pathway. Stk1 can associate with Plk1 in cells and furthermore can phosphorylate Plk1. It can also act on substrates such as myelin basic protein and histone 2A on serine and threonine residues.