MicroRNAs (miRNAs), 18-25 nt single-stranded RNAs, act as regulators in fine-tuning gene function. The absence of miRNAs is not life-threatening in early embryonic development of Dicer-knockout zebrafish and mice, which may account for genetic expression of various traits as a result of miRNA complexity in higher animals during natural evolution. The Pol-II-mediated intronic miRNA is a useful tool to validate the function of computer-predicated miRNAs in zebrafish and mice. (c) 2005 Elsevier Inc. All rights reserved.