miRNAs are a family of approximately 22-nuleotide-long noncoding RNAs involved in the formation and progress of tumors. Since traditional methods for the detection of miRNAs expression have many disadvantages, we developed a simple method called polyA RT PCR. With this method, we detected a series of miRNAs and found that mir-126 is one of the miRNAs underexpressed in breast cancer cells. Flow cytometry analysis showed that mir-126 inhibited cell cycle progression from G1/G0 to S. Further studies revealed that mir-126 targeted IRS-1 at the translation level. Knocking down of IRS-1 suppresses cell growth in HEK293 and breast cancer cell MCF-7, which recapitulates the effects of mir-126. In conclusion, we developed a simple method for high-throughput screening of miRNAs and found that mir-126, a cell growth suppressor, targets IRS-1.