Negative or positive control modes are the most fundamental approaches to gene regulation research. Induction of gene expression can be accomplished by either removing restriction elements (which allow expression from a high-level promoter) or providing stimulatory elements (which promote expression from a low-level promoter) . Transcription factors and microRNAs, a large class of small noncoding RNAs, are the two fundamental mechanisms of gene regulation . A growing number of small RNAs (sRNAs) have been demonstrated to regulate critical pathways in both prokaryotes and eukaryotes . New microRNA families are playing significant roles in the development of novel tissue types and organs, and the regulators themselves appear to be well conserved over extended evolutionary distances for both transcription factors and microRNAs .