Currently, we approach DCIS based on its morphology rather than its etiology. However, morphologically normal-appearing tissue surrounding areas of DCIS may reveal losses of heterozygosity similar to the primary tumor (Lakhani et al. 1995; Stratton et al. 1995; Radford et al. 1995; Fujii et al. 1996). In all likelihood, genetic changes precede morphologic evidence of malignant transformation. We in medicine must learn how to recognize these genetic changes, exploit them, and, in the future, prevent them. DCIS is a lesion in which the complete malignant phenotype of unlimited growth, angiogenesis, genomic elasticity, invasion, and metastasis has not been fully expressed. With sufficient time, most noninvasive lesions will learn how to invade and metastasize. We must learn how to prevent this.