All animal protocols adhered to guidelines by the University of Iowa Institutional Animal Care and Use Committee. Mice harboring loxP-flanked Tcfap2c alleles were obtained from Dr Trevor Williams and maintained as described.14 (link) Mice harboring the MMTV-Neu allele were obtained from Dr Bill Muller. Mice harboring the MMTV-Cre allele were purchased from Jackson Laboratories (Bar Harbor, ME, USA). F4 progeny carrying Tcfap2c allele carrying or lacking MMTV-Cre were crossed with female mice harboring MMTV-Neu. These progeny were used for analysis. Genotyping was carried out with tail snips and analyzed by Transnetyx (Cordova, TN, USA). Tumors (n = 3 tumors) were harvested from mice, formalin fixed and paraffin embedded. Hematoxylin and eosin and IHC analyses were completed with the aid from the University of Iowa Pathology Core Lab with the aid from a veterinary pathologist. Categorical data were analyzed by Fisher's exact test. IHC was analyzed by t-test. Survival curves were analyzed by long-rank test.