MDCK cells infected by oil-treated virus express viral NS1 mRNA, but minimal amounts of NS1 protein. After infection of MDCK cells with oil-treated (1:4,000 dilution) and untreated PR8 virus at 4°C and 37°C for 18 h. Total RNA was extracted from cells and PR8 NS1 mRNA expression was assessed by relative end-point RT-PCR (A). Transcript levels of NS1 normalized relative to the constitutively expressed GAPDH gene (B). Western blot was used to determine NS1 protein expression in the infected cells (C). The membranes were probed with anti-NS1 or anti-actin antibodies. The data depicted are representative of three separate experiments.