First, we measured virus particle formation by the panel of mutant viruses by HA assay. This showed only small fluctuations in particle assembly and release, with even the most replication deficient virus, T32G, showing on average, only a 4-fold drop in HA titre ( Figure 6A ). The copy numbers obtained for the mutant viruses were normalized to that of the WT virus to derive a relative segment copy number:PFU ratio. Data plotted are the mean ± SEM from at least two independent extractions, and for each extraction, the qRT-PCR reaction was performed in triplicate, with the exception of T32G and A78T+G101T, where RNA was extracted from a single rescue (the mean of triplicate determinations is plotted).