The invention relates to avermectin aglycon synthase genes. An isolated DNA containing a DNA sequence encoding the avermectin aglycon synthase domain which is a multifunctional enzyme protein participating in the biosynthesis of polyketide compounds, or its variant having the avermectin aglycon synthase activity; in particular, a functional module and submodule in a sequence originating in Streptomyces avermitilis DNA; a polypeptide or its variant encoded by this DNA or its variant; a vector containing the DNA or its variant; a host cell transformed by the DNA, a modification thereof or the vector; and a process for producing avermectin.