MicroRNAs, known as small noncoding MiRNAs, 19 to 24 nt in length, are important gene regulators and recognized as key players in carcinogenesis. The mechanism lies in that the MiRNAs can conjugate with their targeted mRNA and then lead to the targeted mRNA degradation or repress their translation. Bioinformatic analysis indicates that each MiRNA can regulate hundreds of gene targets and could serve functionally as "oncogenes" or "tumor suppressor genes", and therefore regulate multiple cellular processes relevant to carcinogenesis and cancer progression.