**1. Introduction**

MicroRNAs are a subgroup of small noncoding RNAs containing 18–25 nucleotides, and they do not carry any genetic information for protein expression. They regulate the posttranslational gene expression by binding 3′ untranslated region (UTR) of the target messenger RNA (mRNA). Approximately 30% of protein coding genes are regulated by miRNAs, and they have important roles in cellular functions including proliferation, differentiation, apoptosis, signaling, metabolism, and tumorigenesis. Due to their effect on crucial processes, miRNAs are significant modifiers of transcription and translation of both oncogenes and tumor suppressor proteins. Hence, some of them are classified as oncomiR and tumor suppressor miRNA in the cellular processes of tumor [1].

First miRNA, lin-4, was discovered in *Caenorhabditis elegans* in 1993, and it had role on the regulation of larval development by the repression of a nuclear protein encoded by *lin-14.* The second discovered miRNA, let-7, is expressed in late development and complementary to the 3′ UTR of the several genes including lin-14,

lin-28, lin-41, lin-42, and daf-12. After the discovery of lin-4 and let-7, miRNAs were shown in other organisms including plants and animals [2, 3], and over 10,000 miRNAs have been identified in various organisms. In humans, over 2500 types of encoded miRNAs have been determined [4].
