Abstract
Many attempts are made to identify critical genetic events responsible for the development and progression of breast cancer. There is increasing evidence that breast cancer is a heterogeneous disease, both, phenotypically as well as with respect to its molecular biologically. It is, therefore, extremely difficult to establish a diagnostically and prognostically relevant tumourigenesis model. Emerging new techniques such as microarrays, will provide us with a wealth of additional data over the next years. The precise sampling of tumour material in clearly defined histopathological lesions will be a prerequisite for the assignment of specific genetic alterations to defined stages of breast disease.