 LCIS is a monoclonal pathologic entity which is subject to characterization at the molecular genetic level, and that these molecular genetic alterations may be used to predict the subsequent development of invasive breast cancer. Prophylactic mastectomy specimens from women with multifocal LCIS, and invasive breast cancer specimens which display coexisting LCIS, will be examined for X-chromosome inactivation patterns and loss of heterozygosity to assess for monoclonality. If clonality is present, we will assess for microsatellite instability, and a microarray-based comparative genomic hybridization CGH technique will be used to identify genetic alterations present in LCIS. Lastly, LCIS biopsy specimens from untreated patients who, after follow-up did or did not develop invasive breast cancer, will be evaluated to determine whether the nature or extent of any identified genetic alterations can be correlated with the subsequent development of invasive breast cancer. We hypothesize that a fraction of LCIS lesions will reflect a monoclonal origin, that those lesions of monoclonal origin will display evidence of specific molecular genetic alterations, and that these specific alterations will correlate with the likelihood of the subsequent development of invasive breast carcinoma. 