Discoidin domain receptors (DDRs) are a subfamily of receptor tyrosine kinases, the only members known to respond to an ECM component. DDR2 binds the major fibrillar collagens types I, II, III, and V) and the non-fibrillar collagen X (Shrivastava et al. 1997, Vogel et al. 1997, Leitinger & Kwan 2006). DDR proteins bind collagen as dimers (Leitinger 2003, 2011). DDR2 is confined to mesenchymal cells where it controls developmental processes and regulates cell adhesion, migration, proliferation, and remodelling of the extracellular matrix by controlling the expression and activity of matrix metalloproteinases (Leitinger & Hohenester 2007).