A DNA molecule capable of forming a stem loop structure during transcription into mRNA, the stem loop structure having a ribosome-binding site located therein, the DNA molecule comprising:
- a DNA sequence (a) encoding an RNA sequence designed for a part of the sequence to participate in the formation of the stem loop structure;
- and
- a DNA sequence (b) encoding an RNA sequence complementarily binding to the RNA sequence designed for the part of the sequence to participate in the formation of the stem loop structure, and being located after a stop codon.