132 Espinosa and Slack: The Role of MicroRNAs in Cancer miRNAs. Notably, the majority of human miRNAs are transcribed from regions found within introns of either protein-coding or non-protein-coding transcripts, but a minor- ity is found in isolated regions of the genome, within the exons of noncoding mRNA genes, or within the 3’UTRs of mRNA genes. In the nucleus, these pri-miRNAs are capped and polyadenylated prior to being processed by Drosha, a member of the RNase III enzyme family, in conjunction with the double-stranded RNA-binding pro- tein DGCR8/Pasha. This processing step produces segments ~70 nucleotides in length, which fold into stem-loop structures known as pre-miRNAs [9]. These are ex- ported from the nucleus in a GTP-dependent fashion by exportin 5 and are subject to an additional processing step by another RNase III enzyme, Dicer. This step releases a dou- ble-stranded RNA duplex, ~22 nucleotides in length, which in turn is incorporated into the miRISC complex, in analogous fashion to that observed in RNA interference (RNAi). In this complex, the mature miRNA strand is retained, and the complex is now capable of regulating its target genes. The identification of miRNA target transcripts remains one of the greatest chal- lenges in the field today. MiRNAs can be or- ganized into families based on sequence homology, which is particularly strong at the 5’ end of the mature miRNA, suggesting this section of the mature transcript has been pre- served through evolution and thus plays an important role in the process of target recog- nition. Indeed, studies have shown this 5’ re- gion, often called the “miRNA seed,” to be crucial for both the stability of the mature miRNA and its incorporation into the miRISC complex [10-14]. Bioinformatic ap- proaches have taken advantage of this “miRNA seed” to predict miRNA targets across the genome. It has been predicted that a single miRNA can bind over 200 different target transcripts, and, notably, these targets are highly diverse, from transcription factors to transporters [15-24]. There is nevertheless evidence support- ing a role for the regions not encompassed within the “miRNA seed.” Perhaps the clear- est example of this is found in let-7, whose complete mature miRNA sequence is con- served across species [25], suggesting an important role for the 3’ region of the miRNA in target transcript binding. Further insights into the miRNA-target recognition mechanism are needed, as they will make target prediction much more accurate and ef- ficient. A POTENTIAL ROLE FOR miRNAS IN CANCER Cancers of all types share a number of characteristics, such as the loss of cellular identity, an increased ability to grow and proliferate, and alterations in the systems controlling cell death. Studies performed in a variety of organisms have revealed that miRNAs have the ability to regulate these cellular processes, suggesting that they could be involved in cancer. For instance, lin-4 and let-7 control the timing of devel- opmental events in C. elegans. Mutations in these miRNAs resulted in abnormalities in cell-cycle exit as well as in the execution of a terminal differentiation program, prevent- ing cells from reaching their fully differen- tiated state [2]. In Drosophila, it has been found that over-expression of the Bantam miRNA causes excessive growth of wing and eye tis- sue by blocking the pro-apoptotic action of the gene hid [26, 27]. It was also found that another Drosophila miRNA, miR-14, can act as a strong suppressor of apoptosis. Deletion of this miRNA caused increased expression of the apoptotic effector caspase, Drice, suggesting a direct regulation of this factor by miR-14 [28]. Likewise, the miR-2/6/11/13/308 family of Drosophila miRNAs has been reported to induce wide- spread apoptosis in embryos through the regulation of the proapoptotic factors hid, grim, reaper, and sickle, which act through the inhibition the caspase inhibitor Diap1 in response to a number of natural and toxic conditions [29]. Another aspect in which miRNAs ap- pear to have a role is the proper differentia-