Abstract: The present invention relates to gene regulation cassettes useful for (1) detecting the specific mRNA sequences which best enhance the translation efficiency of mRNA into protein when a specific foreign gene is adjacent thereto and (2) expressing foreign genes in procaryotic cells; expression vectors containing the gene regulation cassettes adjacent to a foreign gene such that the expression of the foreign gene is under the control of the gene regulation cassettes; and microorganisms transformed with the expression vectors. In the former gene regulation cassette, a restriction site encompasses or is adjacent to 2 base pairs 3' of the start of mRNA transcription of the lambda P.sub.R operator/promoter. This gene regulation cassette is useful for inserting and selecting sequences that generate a favorable rate of translation efficiency of the foreign gene. In the latter gene regulation cassette, a lambda P.sub.R operator/promoter is fused to a modified ribosome binding site.