NOVA follows amazing scientific developments in gene therapy, culminating in our ability to make precise edits to human DNA. Find out how the discovery of CRISPR CAS-9 editing, which has shown early promise as a cure for several harmful genetic diseases, also has the capability of making changes to healthy genes – raising ethical questions about whether we should be altering human DNA at all. Pictured: David Sanchez, a teenager with sickle-cell disease, looks at a tube containing the CRISPR gene-editing machinery.