Researchers at Great Ormond Street Hospital in London have developed a better virus to deliver an inserted gene for severe combined immunodeficiency (SCID) patients, one that is self-inactivating and less likely to turn on other genes.

Cancer has been a serious side-effect of SCID gene therapy. Since 2002, four of 10 children in one SCID trial have developed leukemia, apparently because the retrovirus used to insert a curative gene into patients’ blood stem cells turned on a cancer gene.

If the new vector for gene delivery is successful, similar vectors may be used to treat other blood diseases such as sickle cell disease.