Scientists have repurposed the CRISPR-Cas9 system to do all sorts of things. They design guide RNA to match any genes that they might want to remove or change, like a disease-causing mutation in humans or a gene that regulates growth in plants, then attach Cas9. For the sickle cell therapy, they targeted BCL11A in red blood cells. BCL11A has many important roles in the body, but in red blood cells, it represses a protein called fetal hemoglobin. If its disabled, the cells will make fetal hemoglobin—which prevents cells from sickling.