Vascular permeability: the flow factor R ivers rarely run a smooth course. Blood vessels, just like rivers, have turns, tributaries and other obstacles that create eddies and other irregularities in the fl ow. In vessels, such areas of disturbed fl ow are more permeable and more prone to atherosclerotic plaques. Orr et al. (page 719) now reveal that a matrix protein that is abundant at such sit...