Tyrosine-protein kinase (TYRO3) belongs to the Tyr protein kinase family (AXL/UFO subfamily). The UFO family of receptor tyrosine kinases is comprised of subfamily members Rse(also referred to as Tyro3 or Sky) and UFO (also called Tyro7 or Axl). Two distinct isoforms of Rse, designated Brt and Etk-2, have been described. Brt differs from Rse at its C-terminus, but more importantly lacks the N-terminal 31 amino acid signal peptide sequence present in Rse, which is replaced by a 27 amino acid Brt-specific sequence. It has been suggested that as a result of this alternative splicing event, Brt resides in the cytoplasm, unlike Rse which is expressed on the cell surface. Ekt-2 also lacks an N-terminal signal peptide which is substituted with a 45 amino acid Ekt-2-specific sequence. Protein kinases mediate most of the signal transduction in eukaryotic cells, regulating cellular metabolism, transcription, cell cycle progression, cytoskeletal rearrangement and cell movement, apoptosis, and differentiation.