In recent years scientists have been able to correct genetic diseases by removing these stem cells, editing their genomes and then implanting them back into the patient, but that adds complications. Now, new research led by Harvard scientists has successfully edited the genes of stem cells while still in the body.

When it comes to treating genetic diseases, it can be kind of like cleaning up pollution in a river. If you just pick up litter downstream, the river will only get dirty again unless you tackle the problem further upstream. In the same way, treating diseased cells won't help much if you don't address the stem cells, which will quickly replace the healthy cells with new diseased ones.

Currently, fixing stem cells involves removing them from their hideouts deep inside the body, then genetically altering them and putting them back into the patient. There are a lot of potential failure points in that complicated procedure: the stem cells can die in the culture dish, the patient's immune system can reject them once transplanted, or they can just fail to fire back up.