Just one injection could be enough to mean people with haemophilia B no longer need medication, according to an early study in the UK and the US.Six patients were given a virus that infects the body with the blueprints needed to produce blood-clotting proteins. Four of them could then stop taking their drugs.Doctors said the gene therapy was "potentially life-changing".Other researchers have described it as a "truly a landmark study."People with haemophilia B have an error in their genetic code, which means they cannot produce a protein called factor IX, which is critical for blood-clotting.Patients are currently treated with factor IX injections, sometimes multiple times per week, but the manufacturing process is expensive.Researchers at University College London and St Jude Children's Research Hospital in the US were looking for a more permanent solution.