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Medical Definition of Is element
1. Mobile nucleotide sequences that occur naturally in the genomes of bacterial populations. When inserted into bacterial DNA, they inactivate the gene concerned, when they are removed the gene regains its activity. Closely related to transposons and range in size from a few hundred to a few thousand bases, but are usually less than 1500 bases. This entry appears with permission from the Dictionary of Cell and Molecular Biology (11 Mar 2008)