Mouse models have shown a direct in vivo tumour suppressor role for Brca2 in the mammary gland and have demonstrated a synergistic tumour suppressor activity with Trp53. However, Brca2 heterozygous animals do not show a predisposition to tumour formation and Brca2 null mice result in embryonic lethality [18], [22], [23]. This evidence concerns the gene TP53 and neoplasm.