SUM225 cells are resistant to trastuzumab, a HER2-targeted monoclonal antibody, but are sensitive to lapatinib, a small molecule dual RTK inhibitor of HER2 and epidermal growth factor receptor (EGFR).15–17 To examine the differential drug sensitivity of spatially distinct tumor cells in this model, female NOD/scid mice bearing HER2+ SUM225 DCIS-like tumors were randomized into two treatment groups: lapatinib monotherapy (200 mg/kg/day) or vehicle alone for a period of 5–10 days (n = 4–5 mice per group). Here, EGFR is linked to neoplasm.