Mutations in the tumour suppressor gene p53 occur in 20–40% of sporadic breast cancers (Osin and Lakhani, 1999) and are associated with a poor prognosis (Pharoah et al, 1999; Overgaard et al, 2000) and poor response to treatment with certain chemotherapeutic and hormonal agents (Berns et al, 2000; Kandioler-Eckersberger et al, 2000). The gene discussed is TP53; the disease is breast cancer.