According to some published reports, simple repeats can modulate gene transcription. The ability of (AC)n and (AT)n sequences to modulate gene expression in E. coli was investigated. These repeats were inserted in the leader sequence, in its 3' uncoding region, and in both these sites. When simple sequences were inserted in the leader sequence, gene expression was the highest for constructions containing 40 bp insertion. An increase in the insertion length led to a decrease in the gene expression. The (AC)20 sequence insertions in a 3' uncoding site of cat gene did not notably change its expression. Gene expression was much more modified by inserting the simple sequences on both sites of cat gene. Cat gene expression depended on the ratio of simple sequence lengths in its 5' and 3' uncoding sites. It was the highest when (AC)n sequence in the gene leader sequence or its 3' uncoding region was 40 bp. Gene expression modulation by simple sequence is assumed to be associated with the DNA lability increase.