Previous studies have suggested that microRNAs (miRNAs) may play important roles in tumorigenesis, but little is known about the functions of most miRNAs in cancer development. In the present study, we set up a cell-based screen using a luciferase reporter plasmid carrying the whole kb 3¢-UTR of estrogen receptor a(ERa) mRNA cotransfected with a synthetic miRNA expression library to identify potential ERa-targeting miRNAs.