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Regulatory non-protein-coding RNA genes and their transcripts were first found and characterized in bacteria but encompass all biological kingdoms. The complexity of non-coding RNAs (ncRNAs) in terms of number and types increases with degree of biological development, whereby humans and other primates appear to have the largest number. Many regulatory ncRNAs base-pair to a target RNA or DNA and inhibit target function. Bacterial ncRNA genes largely respond to environmental stress conditions and help protect the organism from adverse conditions. The prokaryotic RNAs are for the most part small (