An enhanced version of the CRISPR–Cas9 genome-editing tool can alter the DNA of bone-marrow stem cells, offering a potential treatment for certain blood diseases.

Both sickle-cell disease and the illness β-thalassaemia are associated with misshapen red blood cells, a consequence of mutations in genes encoding the protein haemoglobin. Daniel Bauer at Boston Children’s Hospital in Massachusetts and his colleagues isolated haematopoietic stem cells — which are found in the bone marrow and develop into blood cells — from the blood of people with either β-thalassaemia or sickle-cell disease.

CRISPR–Cas9 has been used to modify stretches of DNA in the genomes of many types of cell, but it hasn’t worked reliably in haematopoietic stem cells. To get around this, the researchers used an altered version of CRISPR’s DNA-cutting component to switch on a healthy haemoglobin gene already present in the stem cells’ genomes.

When the researchers infused the edited cells into mice, the human stem cells produced normal red blood cells, suggesting that the authors’ strategy could eventually provide a treatment for haemoglobin disorders.