To understand why Her2t/w/Adamts18−/− mice are more likely to form tumors and migrate than Her2t/w/Adamts18+/+ mice, we compared the proliferation, migration, and invasion capacity of primary tumor cells isolated from Her2t/w/Adamts18−/− mammary tumors, Her2t/w/Adamts18+/+ mammary tumors, and Her2t/t mammary tumors. The gene discussed is ADAMTS18; the disease is breast cancer.