MicroRNAs (miRNAs) are 20-22 nucleotide small non-coding RNAs that regulate gene expression. The over-expression or mis-expression of many miRNAs has been shown to cause major phenotypic changes. However, a single miRNA deletion seldom generates leads to an obvious phenotype. This suggests that there may be a high degree of redundancy in the miRNA regulating network. To understand how redundant sets of miRNAs regulate gene families in Drosophila melanogaster, we predicted targets of miRNAs in D. melanogaster 3â UTRs and constructed a set of target gene networks. Our results show that larger paralogous gene families as well as genes with longer 3âUTRs are regulated by more numbers and distinct families of miRNAs, which causes higher level of redundancy. Based on these results, we prioritized a set of redundant miRNAs and paralogous gene families for investigation by genetic experiments. We chose miR-7, miR-8 and miR-9 and their target families Rhomboid (Rho) and enhancer split (E(spl)) for knock out investigation. We firstly determined the expression patterns of both miRNAs (miR-7/8/9) and target genes (Rho and E(spl) m8 and m5). We then analysed the impact on heterozygous miR-9 knock out on Rho and m8 and m5 in Drosophila embryonic development. We find that miR-9 clearly impacts on the Rho regulatory network, showing increased expression on miR-9 knock out. However, miR-9 knock out has no significant effect on m8 and m5. We finally conducted tissue specific knock out by CRISPR-Cas9 to sequentially knock out miR-7, miR-8, and miR-9, together with all pairwise combinations and the triplet miR-7/miR-8/miR-9. For single knockouts, we observed a decrease in survival rates. The pairwise knockouts showed further decreased survival, and the triple knockout showed the lowest survival rate of all. The severity of the phenotypic changes therefore correlated with the number of miRNAs knocked out. Our results strongly support the redundant action of miR-7, miR-8 and miR-9 on their gene targets. We show that phenotypic changes increase on removal of miRNA redundancy, and provide an explanation for the observation that single miRNA gene knockouts do not generally exhibit obvious phenotypes.