Whenever a blood vessel is broken due to injury, bleeding occurs. Bleeding soon stops, however, because the break is quickly sealed by a clot. The process by which a clot forms involves many steps and more than a dozen different proteins, or clotting factors. First, platelets adhere to the connective tissue exposed by the break in the vessel's lining. Then, the platelets and the damaged cells secrete a variety of clotting factors. These participate in a cascade of chemical reactions of which the end result is the formation of strands of fibrin, the basis of the clot. Because the factors act like links in a chain, if any one of the genetically controlled protein clotting factors is absent or deficient, the blood will clot too slowly or not at all.

Two of the disorders involve platelets. In both...