The UFO family of receptor tyrosine kinases is comprised of subfamily members Rse (also designated Tyro3, Sky, Brt, Dtk, Etk2 and Tif), Axl (also designated UFO or ARK) and Mer (also designated Nyk or Eyk). Rse is expressed Preferentially in the adult brain with lower expression in other tissues. Axl is found at highest levels in heart and skeletal muscle. Mer has been identified as a tyrosine kinase is involved in the development of glioblastomas. It is expressed at highest levels in ovary, prostate , Lung and kidney. Gas6, a growth arrest specific gene, and the related anticoagulation factor Protein S, have been identified as ligands for the UFO family of receptors.