We performed a fluorogenic assay for neuraminidase (NA) of influenza, where 2′-(4-methylumbelliferyl)-α-D-N-acetylneuraminic acid (MUNANA) is hydrolysed to 4-methylumbelliferone (4-MU) and acetylneuraminic acid by NA activity (Fig. 1a). This evidence concerns the gene XK and influenza.