Gene therapies replace or modify disease-causing genes in human cells. Those therapies, which are still relatively new, are used to prevent, cure, or treat the symptoms of a particular disease. In this report, CBO discusses how it would estimate the budgetary effects of policies that sought to increase the use of gene therapy treatment for sickle cell disease (SCD). CBO focuses on SCD because two new gene therapies were recently approved for its treatment, and the first few patients outside clinical trials have been treated with those therapies. Gene therapies in development target a range of diseases, including cancer, heart disease, and inherited genetic diseases.