Abstract MicroRNAs (miRNAs) are a group of small non-coding RNAs involved in several important biological processes. Recent studies have implied that imprecise cleavage of primary or precursor RNA by Drosha or Dicer respectively could produce miRNAs with variations at 5’-and/or 3’-ends, which were termed “isomiR”. The existence of isomiRs may increase the complexity of miRNA biology, although their functions have not been explored. This is further complicated by the observation of variations in the population of isomer for a given miRNA among different species as well as among different types of cells within the same species (human cells), which raises the issue whether these isomers might exert differential effects on target gene regulation. Here, we utilized three miR-31 isoforms (miR-31-H, miR-31-P, and miR-31-M) which differ from each other only slightly in their sequences at both ends, to compare their capacity to regulate a predicted target, Dicer. Interestingly, we found only miR-31-P was able to directly repress Dicer expression in both breast cancer MCF-7 cells and lung cancer A549 cells, resulting in their enhanced sensitivity to cisplatin, which is known to be associated with Dicer knockdown. In line with this, the amount of miR-31-P bound to the RNA-induced silencing complex (RISC) was greater than that of miR-31-H and -M by RNA-CHIP analysis. Furthermore, evaluation of other known miR-31 target genes showed that expression of some target genes was indeed differentially regulated by miR-31 isoforms. Our findings not only revealed Dicer to be a direct target of miR-31, but also demonstrated the disparate functions of isomiRs for the fist time, providing a novel functional mechanism of fine-tuning of gene expression by miRNAs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3983. doi:10.1158/1538-7445.AM2011-3983