These findings, along with the description of germline mutations and deletions of PTEN in two hereditary cancer predisposition diseases (Liaw et al, 1997; Marsh et al, 1997; Nelen et al, 1997), i.e. Cowden Disease and the Bannayan-Zonana-syndrome, point to a role of PTEN as a tumour suppressor gene in the pathogenesis of malignant tumours. The gene discussed is PTEN; the disease is Cowden disease.