ETK, also known as BMX, is a member of the Bruton's tyrosine kinase (Btk) family. It is expressed in a variety of hematopoietic, epithelial and endothelial cells. Etk, like other Btk family members, contains a pleckstrin homology (PH) domain and Src homology SH3 and SH2 domains. It participates in multiple signal transduction pathways. Phosphorylation of Tyr566 by Src kinase is required for activation of Etk in vivo. In endothelial and epithelial cells, Etk is regulated by FAK through phosphorylation at Tyr40.