Using MCF10A cells overexpressing wild type ERBB2 (MCF10A-ERBB2), mammary tumor epithelial cells derived from MMTV-neu transgenic mice (MTEC-Neu) and SK-BR3 cells, all of which overexpress similar levels of ERBB2 (Figure S1A), we examined the effects of lapatinib treatment under normoxic and hypoxic (1% O2) conditions. The gene discussed is ERBB2; the disease is breast cancer.