ɔːlðˈoʊ ɪt hɐdbɪn nˈoʊn fɔːɹ mˈoːɹ ðˌænə dˈɛkeɪd ðæt ˌɪnfluːˈɛnzə vˈaɪɹəsᵻz wɪð pˈɑːɹʃəl dᵻlˈiːʃənz ɪn ˌɛnˈɛs wˈʌn pɹˈoʊtiːnz wɜːɹ ɐtˈɛnjuːˌeɪɾᵻd, ˈɔːl bˌʌt wˈʌn ˌɛnˈɛs wˈʌn tɹʌŋkˈeɪʃən vˈɛɹiənts ʌv ˌɪnfluːˈɛnzə ɐ vˈaɪɹəsᵻz wɜː dʒˈɛnɚɹˌeɪɾᵻd baɪ ɪn vˈɪtɹoʊ mjˌuːɾeɪdʒˈɛnᵻsˌɪs.
Although it had been known for more than a decade that influenza viruses with partial deletions in NS1 proteins were attenuated, all but one NS1 truncation variants of influenza A viruses were generated by in vitro mutagenesis.