Haemophilia B is an X-linked recessive bleeding disorder, arising from a deficiency of coagulation factor IX. It has been a target for gene therapy ever since the factor IX gene was cloned in 1982. Several distinct approaches have been evaluated in humans over the last 30 years, but none has resulted in tangible corrections of the bleeding phenotype in humans until recently. Our group has now shown that lasting clinical improvement of the bleeding phenotype in patients with haemophilia B is possible following a single systemic administration of a self-complementary adeno-associated virus vector to deliver an optimised factor IX expression cassette to the liver. Success in this trial raises hope for patients with severe haemophilia B as well as others with inherited monogenetic disorders of the liver where current treatment options are limited.