MicroRNAs (miRNAs) represent a class of small non-coding RNAs regulating gene expression by inducing RNA degradation or interfering with translation. Aberrant miRNA expression has been described for several human malignancies. Herein, we show that miR-145 is down-regulated in human cancer cell line MCF-7 when compared to normal human mammary epithelial cell line MCF10A. Overexpression of miR-145 by plasmid inhibits MCF-7 cell growth and induces apoptosis. Subsequently, RTKN is identified as a potential miR-145 target by bioinformatics. Using reporter constructs, we show that the RTKN 3' untranslated region (3'UTR) carries the directly binding site of miR-145. Additionally, overexpression of miR-145 in MCF-7 reduces RTKN protein expression as well as mRNA level. Furthermore, down-regulation of RTKN by siRNA can inhibit MCF-7 cell growth. Taken together, we propose that loss of miR-145 may provide a selective growth advantage for MCF-7 by targeting RTKN.