This recombinant TRAIL gene (sTRAIL) was composed of the soluble form of the human Flt3L gene (hFlex) at the 5′ end and the human TRAIL gene at the 3′ end (aa residues 95–281) with an isoleucine zipper at the N-terminal of TRAIL, which was previously shown to significantly enhance trimerization of the fusion protein as well as its anti-tumor activity