. "Combining with a signal and transmitting the signal from one side of the membrane to the other to initiate a change in cell activity by catalysis of the reaction: ATP protein serine = ADP + protein serine phosphate, and ATP + protein threonine = ADP + protein threonine phosphate." . . . "STK13" . "transmembrane receptor protein serine/threonine kinase activity" . "activin receptor kinase activity" . "ATP:receptor-protein phosphotransferase activity" . "receptor protein serine/threonine kinase activity" . "receptor serine/threonine protein kinase activity" . "receptor type I serine/threonine protein kinase activity" . "receptor type II serine/threonine protein kinase activity" . "TGF-beta kinase activity" . . . . .