Etk is a member of the Bruton’s tyrosine kinase family. Etk is expressed in a variety of hematopoietic, epithelial and endothelial cells. It participates in multiple signal transduction pathways. Phosphorylation of tyrosine 566 by Src kinase is required for activation of Etk in vivo. In endothelial and epithelial cells, Etk is regulated by FAK throμgh phosphorylation at tyrosine 40.