The enhanced permeability and retention (EPR) effect is supposed to help nanoparticles accumulate in tumors. Because of their size, the particles cannot slip between endothelial cells lining normal blood vessels (top). But tumor tissue often contains leaky vessels that allow nanoparticles to sneak through (bottom), according to EPR theory. Unlike in healthy tissue, tumors lack efficient drainage by the lymphatic system, and this slows the clearance of nanoparticles.