(A) 10B6 (200 μg/mouse) was i.p. injected twice weekly into PyMT mice from 9 to 13 weeks of age. Tumor growth (mean ± SEM; n = 4–6 per group) was determined by weekly caliper measurements. *P < 0.05, **P < 0.01, ***P < 0.001 vs. IgG control, 2-way ANOVA. (B) Primary mammary epithelial cancer cells were isolated from 12-week-old FP635/PyMT and FP635/PyMT/ETP mice and implanted into WT recipients with the same volume of Matrigel. For the 10B6 group, 10B6 was added in a Matrigel plug (10 μg/plug) admixed with ETP⁺-cancer cells (i.e., ETP⁺/10B6). A representative whole-body image was acquired 25 days after implantation using IVIS fluorescence scanner. Artificial color indicates fluorescence signal intensity accounts for tumor volume (AU). Quantitative results are represented as mean ± SEM (n = 3 per group). *P < 0.05, unpaired t test. (C–I) 6 weeks after implantation, tumor tissues were excised from Ctrl-, ETP⁺-, and ETP⁺/10B6-cancer cells allografted mice and stained for H&E (C), Masson’s Trichrome C (D), α-SMA (E), FSP-1 (F), CD31 (G), F4/80 (H), and Ki67 (I). Quantified results in D–I are mean ± SEM (multiple images, n = 3 per group). *P < 0.05, **P < 0.01, unpaired t test. Scale bars: 50 μm. (J) Working model for ETP in mammary tumor progression.