Overexpression of cyclin E is strongly linked to an aggressive phenotype and poor prognosis in breast cancer. Unexpectedly, it has been shown that cyclin E overexpression decreases mobility and invasiveness of breast cancer cells. In fact, in a study of 985 breast cancers, cyclin E correlated with less infiltrative growth. This suggests a novel function for cyclin E in breast cancer tumorigenesis and a mutual control of proliferation and invasive behavior.