Gong et al. (19) found increased miR-21 ISH signal in resistant tumors before and after trastuzumab-directed therapy, and showed that miR-21 antisense oligonucleotides could restore trastuzumab sensitivity in resistant cells by inducing PTEN expression, suggesting that miR-21 mediates trastuzumab resistance and that antagonizing miR-21 in a therapeutic setting could sensitize cancer cells to HER2-directed therapy. This evidence concerns the gene ERBB2 and cancer.