Endothelial cells form a barrier, the integrity of which is mediated by integrins, tight junctions, VE cadherin, FAK, and the Src kinase Lyn. Loss of Lyn affects FAK’s ability to form adhesions and may also contribute to tight junctions. When the integrity of the endothelial barrier is compromised by injury, vascular leakage into the extracellular matrix or surrounding tissues occurs.