MicroRNA (miRNA) are a class of non-coding RNAs found both in normal tissues and cancer cells. The natural miRNA, which target cancer genes for transcriptional repression are still unknown. Approaches for synthetic miRNA design targeting cancer genes have not yet been established. We designed miRNAs targeting the 3' UTR (nt 4350-4372) of HER-2 proto-oncogene. One miRNA (miR-14U) was designed by introducing a mutation in the nt 14 counting from the 5' end of anti-sense RNA. Two others (miR4350-10GGA and miR4350-11AAGCU) were designed by introducing either loop-forming nucleotides in position 10 or a part of the complementary sequence of the Brd-box consensus sequence, in position 11. miR4350-10GGA was more effective than the anti-sense strand in decreasing the numbers of ovarian tumor SKOV3 cells, which over-expressed HER-2 protein. Its inhibitory effects were lower than that of corresponding double-stranded (ds) RNA4350-4372. Inhibition of HER-2 expression mediated by miRNAs was higher in cells expressing higher levels of HER-2 protein than in cells expressing lower levels of HER-2 protein. This is the first demonstration of inhibition of expression of a constitutively over-expressed tumor protein by designed synthetic miRNA and its cor-responding dsRNA targeting 3' untranslated regions in mRNA. Our results also show that measuring the effects of miRNA and dsRNA in pooled populations of tumor cells, which express various levels of target oncogenes, may reflect the survival responses of siRNA resistant tumors rather than the growth inhibitory responses of siRNA-sensitive tumors.