Targeting the third Ig-like domain of nectin-4, which cis-interacts with ErbB2, might be an alternative strategy for the treatment of trastuzumab-resistant breast cancers expressing p95-ErbB2 and ErbB2ΔEx16, although it was recently reported that trastuzumab is effective for breast cancers expressing ErbB2ΔEx1621,22. This evidence concerns the gene NECTIN4 and breast cancer.