Several receptor tyrosine kinases generate soluble ligand binding domains either by
differential splicing resulting in a truncated RNA transcript, or by proteolytic cleavage.
Although the exact role in vivo of these soluble extracellular domains is unclear, proteolysis
may function to down-regulate the receptor, and soluble extracellular domains (ECD) may
compete with the intact receptor binding to ligand. Axl is a member of a new class of receptor
tyrosine kinases characterized by an ECD resembling cell adhesion molecules and unique …