On the left, F.IX structure derived from 1RFN PDB (Protein Data Bank) entry. Residue R338 (spacefilling) was mutated to A for gene transfer. On the right, F.IX clotting activity and blood loss in a tailclipping assay 4 weeks after delivery of AAV-wild type F.IX or AAV-R338A F.IX to hemophilia B mice. Activity is reported as percentage of normal mice. OD indicates absorbance of hemoglobin in the saline solution in which the tail was placed.