U2AF1 belongs to the splicing factor SR family of genes. U2 auxiliary factor, comprising a large and a small subunit, is a non-snRNP protein required for the binding of U2 snRNP to the pre-mRNA branch site. This protein is the small subunit which plays a critical role in both constitutive and enhancer-dependent RNA splicing by directly mediating interactions between the large subunit and proteins bound to the enhancers. Recombinant human U2AF1 protein, fused to His-tag at N-terminus, was expressed in E.coli.