Serine/threonine-protein kinase involved in regulation of lymphocyte migration. Phosphorylates MSN, and possibly PLK1. Involved in regulation of lymphocyte migration by mediating phosphorylation of ERM proteins such as MSN. Acts as a negative regulator of MAP3K1/MEKK1. May also act as a cell cycle regulator by acting as a polo kinase kinase: mediates phosphorylation of PLK1 in vitro; however such data require additional evidences in vivo. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inhibited by the pyrrole-indolinone inhibitor SU11274 (K00593): intercalates between the ATP-binding Lys-65 and alpha-C glutamate (Glu-81), resulting in a partial disordering of the lysine side chain. Also specifically inhibited by erlotinib. Slightly inhibited by gefitinib. Homodimer; homodimerization is required for activation segment autophosphorylation. Highly expressed in rapidly proliferating tissues (spleen, placenta, and peripheral blood leukocytes). Also expressed in brain, heart, skeletal muscle, colon, thymus, kidney, liver, small intestine and lung. Autophosphorylates following homodimerization, leading to activation of the protein. The disease may be caused by variants affecting the gene represented in this entry. Inhibition by erlotinib, an orally administered EGFR tyrosine kinase inhibitor used for treatment, enhances STK10-dependent lymphocytic responses, possibly leading to the aggravation of skin inflammation observed upon treatment by erlotinib. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily. Truncated N-terminus.