Bleeding from a normal vessel (top) is quenched by a platelet plug stabilized by fibrin strands. Bleeding continues from a hemophilia vessel (middle) because TFPI blocks factor Xa production leaving inadequate factor Xa to drive fibrin formation or platelet activation. Aptamer ARC19499 inhibits tissue factor pathway inhibitor (bottom) enabling the factor VIIa-tissue factor complex to produce sufficient factor Xa to stabilize the platelet plug. Professional illustration by Paulette Dennis.