In a panel of six HER2-amplified breast cancer lines, HER3 siRNA was as effective as HER2 siRNA at inhibiting proliferation of four lines (BT474, HCC1419, SKBR3, and ZR75-30), and more effective than HER2 siRNA in one of the lines (HCC1954) [158]. This evidence concerns the gene ERBB3 and breast cancer.