The present invention relates to polynucleotide molecules containing nucleotide sequences encoding aveC gene product, and these polynucleotide molecules can be used to change the ratio or number of avermectins of classes 2: 1 produced in S. avermitilis fermentation cultures. In addition, the present invention relates to vectors, host cells and mutant strains of S. avermitilis, in which the aveC gene is inactivated or mutated in such a way as to change the ratio or number of classes 2 avermectins produced. 1. The international application search.
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