Potent, selective inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase (IC50 = 3 nM). Displays > 1000-fold lower potency against ErbB2 (IC50 = 3.4 μM) and a range of other related tyrosine kinases (IC50 > 10 μM). Oral administration inhibits growth of established human xenografts in athymic mice.