To further confirm the specificity of the a 1 -AT variants, we used fowl plague virus (FPV), which contains a hemagglutinin with a multibasic cleavage motif recognized by furin [57] . Thus, the furin-adapted a 1 -AT should prevent virus spread of FPV, while virus spread in the presence of S1P-specific a 1 -antitrypsins should remain unaffected. Fig. 5B clearly demonstrates that the most potent S1P-specific a 1 -AT variant RRIL had no effect on FPV replication, and that virus spread was found to be similar to that observed in wild type CHO-K1 Tet-On cells. In contrast, in cells expressing the furin-adapted a 1 -AT variant RVKR virus spread of FPV was drastically reduced, whereas FPV replication occurred efficiently under doxycycline-free conditions in these cells. These results demonstrate that the generated a 1 -AT variants exhibit high specificity for the corresponding proteases, which are essential for virus spread in cell culture.