In the endocytosis assay, trastuzumab was inefficiently internalized over a period of 4 hours. In MCF7 or MCF10A breast epithelial cells, introduction of HER2L755S, HER2V777L, or HER2L869R did not significantly affect internalization following trastuzumab labeling, or after stimulation with EGF or neuregulin....MCF7 cells expressing HER2L755S, HER2V777L, or HER2L869R were not sensitive to T-DM1 in 2D cell viability assays or 3D Matrigel assays.