Tumor heterogeneity is another consideration: in HER2-positive breast cancer, in contrast to patients with homogeneous HER2 expression, patients with high levels of intratumor or inter-tumor heterogeneity have inferior responses to ado-trastuzumab emtansine (TDM1, trastuzumab-DM1)19. The gene discussed is ERBB2; the disease is breast carcinoma.