FIG 1.

Outline of the experimental design. (A) Influenza virus stocks were generated from reverse genetic clones: H1N1p (blue), H3N2hu (yellow), and H3N2ca (green). (B) The enzyme CMAH catalyzes the enzymatic conversion of Neu5Ac to Neu5Gc. Inactivation of the cmah gene in a C57BL/6 mouse background generates a Neu5Ac-only mouse. (C) Experimental passages of viruses were performed by nasal inoculation of culture-derived virus or lung homogenates into control C57BL/6 (black) or CMAH^−/− (gray) mouse cohorts, 3-day incubation, and harvest of lung homogenates. C57BL/6 mice display Neu5Gc, as a proportion of total Sia, at 45% in trachea and 60% in the lungs. CMAH^−/− mice contain no Neu5Gc in their trachea or lungs.