After decades of study, gene therapy is showing exciting possibility for the treatment of hemophilia B patients. Researchers at Spark Therapeutics have reported promising results in a study of four hemophilia B patients who were injected with viruses containing the gene that codes for factor IX. Factor IX protein which clots blood is otherwise absent in the bodies of hemophilia B patients. While the small study size is keeping scientists from declaring an overall cure, the consistently positive results observed in each of these four patients is being received by scientists as quite encouraging for future one-time treatment possibilities for hemophilia B. In addition to the optimism regarding a potential cure for segments of the hemophilia population, these findings also have encouraging implications for the treatment of several other rare diseases.