miR-128 is more highly expressed in drug-resistant breast cancer samples than in drug-sensitive samples. We have confirmed that Bax is the target of miR-128 by negative post-transcriptional regulation. miR-128 and Bax were detected in the breast cancer cell line, MDA-MB-231, which was then transfected with miR-128 MIMIC (precursor of miR-128) or AMO (antisense-miR-128 oligonucleotides). After transfection, the chemosensitivity of MDA-MB-231 cell was up-regulated with increasing of Bax and inhibition of miR-128. © 2013 International Federation for Cell Biology.