Neoplastic tissue composition and HER-2 expression in small and large tumors. Quantification of tissue composition in small (A) and large (B) tumour xenografts in untreated (CTRL) and Trastuzumab or T-DM1 treated mice (*p < 0.05, vs control) was evaluated as described in Methods Section. C-H: Immunoperoxidase staining of xenografts by anti-HER-2 antibodies. The sharp difference in HER-2 expression (brownish) by neoplastic cells composing small (C) and large (D) tumours is apparent. E and G illustrate sections of small T-DM1 treated tumour xenografts in which black rectangles include a microscopic field shown at higher magnification in F and H, respectively, to document giant mitotic figures (✽) on HER-2 labeled cells. Arrows indicate polynucleated HER-2 positive neoplastic cells. The lower panels show the specific immunofluorescent labeling of metaphase chromosomes (I) by phospho-Histone H3 (PH-H3, green, J) on a large cytokeratin (CK, red, K) positive cell. Arrow points to a giant polynucleated neoplastic cell. Scale bars: C, D = 100 μm; E, G = 50 μm; F, I, L, M = 20 μm and H = 10 μm.