The two viruses were used to co-infect triplicate wells of either MDCK or HAE cells at defined ratios and after 72 h released virus was analysed for the presence of H275Y mutation. Pyrosequence analysis showed that in MDCK cells there was no growth advantage for virus with either H or Y at residue 275 ( Figure 2 ). The output virus from all three wells of MDCK cells contained the same mixture of wild-type and mutant genomes as the input. Thus, the mean percentage of wild-type genotype (H275) following inoculation of the 50: 50 mixture was 50.3% and after inoculation of the 80 : 20 mutant:wild-type mixture this value was 21.4%. In contrast, after 72 h of propagation in HAE cells where mucus could accumulate, the wild-type oseltamivirsusceptible H275 genotype accumulated to a higher degree in two out of three wells than did the drug-resistant variant (Figure 2c) . The mean percentage of wild-type genome in the mixture after 72 h was 59%. The standard deviation of the pyrosequencing assay using this primer set was calculated by performing triplicate runs on six different extracted mixtures and was measured at ,1% (data not shown). Thus, the observed enrichment of wild-type genome after propagation in HAE cells was not a result of assay variation but rather indicative of a selective advantage of H275 in this system.