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.
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.