ERBB2 and neoplasm: To determine whether stromal PTEN deletion exerts stable pro-tumorigenic effects on neighboring mammary epithelium, control (ErbB2;Ptenfl/fl) and experimental (ErbB2;Fsp-cre;Ptenfl/fl) mice were sacrificed prior to exhibiting observable neoplasia (~8–10 weeks of age) and their mammary epithelium was injected at a limiting dose (1 × 105 cells) into the mammary fat pads of wild-type syngeneic recipients and monitored for tumor development.