Scientists in the US say they have successfully corrected a genetic mutation that causes the blood disorder sickle cell anaemia.
The red blood cells are scythe shaped in sickle cell anaemia
A team from University of California, San Francisco carried out the repair on stem cells taken from embryonic mice.
Sickle cell, which primarily affects people of African, Mediterranean, Middle Eastern and Indian descent, can cause circulation problems.
The study features in Proceedings of the National Academy of Sciences.
Sickle cell is caused by a genetic mutation which results in an abnormal form of haemoglobin, the molecule within red blood cells which carries oxygen.
This causes the blood cells to assume a sickle shape and stick together, which means blood cannot circulate properly.
The San Francisco team worked on embryonic stem cells carrying the human sickle cell mutation.
They inserted a healthy copy of the haemoglobin gene, replacing the mutated version.
This insertion generated stem cells carrying sickle cell trait, but not full-blown sickle cell.
Individuals with sickle cell trait are generally healthy.
The researchers eventually hope to be able to genetically alter human embryonic stem cells from a patient's own DNA and transplant them back into the patient, correcting their sickle cell anaemia.
They said that in theory the same stem cell gene therapy could be used to cure another genetic blood disorder, thalassaemia.