On the cover, the stepwise recognition of the protein substrate (S) by prothrombinase (E) that leads to thrombin formation is superimposed on a scanning electron micrograph of a whole blood clot (work of Yuri Veklich and John Weisel). The unique specificity of prothrombinase arises from dominant contributions of exosite binding to substrate affinity rather than simply lock-and-key type active site docking assumed to explain specificity in coagulation. For details see the article by Orcutt et al., pages 46191-46196.