Potent, irreversible human epithelial growth factor receptor 2 (ErbB2) inhibitor (IC50 = 6 nM) that displays > 4000-fold selectivity over EGFR, FGFR, PDGFR, JAK1 and Src. Exhibits potent antiproliferative effects in ErbB2-overexpressing cancer cell lines (IC50 = 5 nM in BT474 breast cancer cells) and significantly inhibits bladder, breast and prostate cancer xenograft growth in vivo.