CR-1–31-B in combination with palbociclib is synergistic in suppressing ER+ breast cancer growth in vivo. A, Tumor growth curves of MCF-7 orthotopic xenografts in nude mice treated for 28 days with vehicle (n = 5), palbociclib (45 mg/kg; n = 4), CR-1–31-B (0.35 mg/kg; n = 4), or palbociclib + CR-1–31-B combination (n = 5). Error bars, SEM values. ****, P < 0.0001; by two-way ANOVA. B, Weight of nude mice over xenograft period. Mice were weighed at the beginning of drug treatment and after day 13, every 2 days thereafter. C, IHC staining was performed on formalin-fixed, paraffin-embedded xenografts tumors at endpoint. Representative results are shown. Scale bar for pRB S807/811, cyclin D1, CDK4, cyclin E1, and Ki67: 60 μm. D, H-scores of cyclin D1, CDK4, cyclin E1, and pRB IHC staining of xenograft tumors and percentage of Ki-67-positive cells in each treatment condition. **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; by Student t test, unpaired, two-sided.