Combination therapy of doxorubicin and GU81 reduces the growth index of MMTV-PyMT tumors. Tumor sections from each treatment group were evaluated for phospho-histone-3 (A&B) and active-caspase-3 (C&D) by immunohistochemistry as described in the methods section. Signal intensity was quantified using Elements software and is displayed and mean +/- SEM. All quantification includes 3 animals/group and 5 sections/animal. D) A growth index was calculated whereby the number of phospho-histone-3 positive cells (actively proliferating) was divided by the number of active caspase-3 positive cells (undergoing apoptosis). *, p < 0.05; **, p < 0.01 vs control, 1-way ANOVA and Mann Whitney test.