Xu et al (1999a) used transgenic mice expressing Cre from the whey acidic protein (WAP) or mouse mammary tumour virus (MMTV) promoter to induce mammary-specific recombination of the Brca1F11 alleles. In both models, different types of mammary tumours developed with a long latency and these tumours showed genomic instability and altered Trp53 expression. The vast majority of these tumours were negative for ER, but a large proportion overexpressed ERBB2 (Table 1). Removal of one Trp53 allele significantly reduced mammary tumour latency (Brodie et al, 2001). This evidence concerns the gene ESR1 and breast cancer.