Animal care and use were in accordance with institutional guidelines. Female, FVB/N-TgN (MMTVneu)-202Mul mice (TgMMTV-neu) (7 weeks old; median weight: 18 g, range: 15.4-18.5 g; Jackson Laboratory) or FVB-Tg(C3-1-TAg)cJeg/Jeg (C3(1)-Tag) mice (6 weeks old; median weight: 18.5 g, range: 16.5-18.5 g; provided by Dr. Jeff Green, NCI) were used in this study. Tumors derived from the C3(1)Tag mouse exhibit a basal phenotype and those derived from the TgMMTV-neu mouse exhibit a luminal/neu phenotype (16 (link)). The mouse mammary tumor cell lines MMC and M6 were derived from spontaneous mammary tumors from TgMMTV-neu (17 (link)) and C3(1)-Tag (18 (link)) mice, respectively. Both cells lines were authenticated before use. The MMC cell line was verified to express rat neu by flow cytometry and the M6 cell line was verified to express the SV40 antigen by Western blot and the estrogen receptor by RT-PCR.