The study of breast cancer has involved comparative pathology since the 1890s. Molecular biology has allowed the detailed examination of human, mouse, and rat genomes. The genes that are associated with breast cancer in humans cause cancer in genetically modified mice and rats. Global genomic analyses demonstrate even broader genomic homologies. The pathology illustrates the principle of One Medicine. This publication examines these contributions and opens the way for the examination of other mammalian species. The development of a new discipline, Genomic Pathology, can now be anticipated.