Two genes for human immune interferon (hIFN-gamma), synthetic (GIFs) and cDNA prepared from mRNA (GIFm), have been expressed in E. coli under the control of the trp promoter. The expression level of GIFs was 100-fold lower than that of GIFm. Secondary structure analysis of two genes' mRNA surrounding Shine-Dalgarno (SD) sequence showed the presence of a 8 base-pair stable stem structure preceding SD sequence in GIFs mRNA, but not in GIFm mRNA. Expression of a GIFs gene, in which the stem structure was rendered unstable by changing two bases, became 100 times greater than that of the original GIFs.