Inhibitory effect of mAbs 1D1 and 1G8 on NA activity of H9N2 X1virus and mAb escape mutants. The inhibition was measured with Mu-NANA assay (A and B) and ELLA
(C and D). m1D1 is the escape mutant of X1 selected with mAb 1D1, while m1G8-1 and m1G8-2 are escape mutants of X1 selected with mAb 1G8.