Do you know why they're having the marrow synthesise fetal hemoglobin versus hemoglobin A(2)? (Is it simply because HbF is one molecule while HbA and HbA2 are two?)
Fetal hemoglobin has a lot of biochemical advantages for fighting sickle-cell disease on its own so this is leveraged in the CRISPR solution - e.g. create more of the cells that inhibit the disease in the first place.