New molecular techniques have yielded a battery of potential therapies for many diseases, but the clinical fruits of these methods have not been abundant, in part because in vitro models used for screening often do not duplicate in vivo conditions. For example, in vivo the microvascular wall prevents many intravenous agents with in vitro activity from reaching target cells in the tissue. Moreover, the magic bullet of cancer immunotherapy — antibodies conjugated to a drug or toxin — do not consistently hit their mark, because the bull's-eye on the other side of the vascular endothelium remains largely inaccessible to the . . .