To evaluate the phenotypic effects of mutp53 in heterozygosity, we established stable cell lines from mouse mammary tumors of +/+;ErbB2, H/+;ErbB2, H/−;ErbB2 (R172H/p53null;ErbB2), −/+;ErbB2, and −/−;ErbB2 genotype (three biological replicas per genotype) (Fig. 1a). Here, ERBB2 is linked to breast cancer.