MicroRNAs (miRNAs) are endogenous small noncoding RNAs (20-23 nucleotides) that negatively regulate the gene expressions at the posttranscriptional level by base pairing to the 3' untranslated region of target messenger RNAs. Hundreds of miRNAs have been identified in humans and evolutionarily conserved from plants to animals. It is revealed that miRNAs regulate various physiological and pathological pathways such as cell differentiation, cell proliferation, and tumoriogenesis. By the computational analysis, it is predicted that 30% of protein-encoding genes are regulated by miRNAs. In this review, we discuss recent remarkable advances in the miRNA biogenetic and functional mechanisms and the involvements of miRNAs in cell differentiation, especially in hematopoietic lineages, and cancer. These evidences offer the possibility that miRNAs would be potentially useful for drug discovery.