Gene therapy is one potential novel therapeutic avenue for the treatment of inherited monogenic disorders. Diseases of the blood are frequent targets for gene therapy because it is relatively easy to harvest haematopoiesis stem cells (HSCs) from the bone marrow, genetically modify the cells ex vivo, and then re-administer the corrected cells back into the patient via intra-venous injection. In this Closeup, Milsom and Williams discuss the work of Roselli et al, who describe the pre-clinical evaluation of the treatment for beta-thalassemia in erythroid cells via the genetic correction of patient HSCs using a lentiviral vector.