Streptomyces sp.139 producted a novel exopolysaccharide (EPS) designated as Ebosin,which has antagonist activity for IL-IR in vitro and remarkable anti-rheumatic arthritis activity in vivo.The biosynthesis gene cluster (ste) of Ebosin consisting of 27 ORFs was identified.In this study for functional analysis of ste26,the gene was disrupted with a double crossover via homologous recombination The molecular weight of EPS-26m produced by the mutant strain Streptomyces sp.