Hemophilia is a genetic disease that impairs the normal process of blood clotting and results in uncontrolled external and internal bleeding. The reader of this book will learn how a diagnosis of hemophilia is made by blood clot-ting tests and measurements of clotting factor levels in blood. The book describes how hemophilia A and B are caused by mutations in genes that encode clotting factor VIII and clotting factor IX, respectively, both of which are carried on the X chromosome. As a result, almost all children born with hemophilia A and B are boys. Hemophilia C is caused by mutations in the clotting factor XI gene on chromosome 4, and occurs in males and females with equal frequency. The author details the use of factor replacement therapy to treat hemophilia, and evaluates the prospects for curing hemophilia through gene therapy and genome editing.