3.5. MicroRNA(miRNA) Array
In 1993, Lee et al. [47] discovered that lin-4, a gene known to control the timing of Caenorhabditis elegans larval development, does not code for a protein but instead produces a pair of small RNAs approximately 22 and 61 nt in length. The shorter lin-4RNA is now recognized as the founding member of an abundant class of short regulatory RNAs called microRNAs or miRNAs [48,49,50]. The importance and the role of miRNA-directed gene regulation are coming into focus as their regulatory targets and functions [51]. Liu et al. [52] described the using of the first miRNA microarray. After that, the miRNA microarrays revealed their functions in control of cell proliferation, cell death associated with carcinogenesis [53,54], fat metabolism in flies [55,56], and modulation of hematopoietic lineage differentiation in mammals [57]. For example, miR-21 was detected by miRNA array in various cancers [58], and high miR-21 expression is associated with the poor survival and poor therapeutic outcome in colon cancer [59].