ESR1 and breast carcinoma: However, in breast cancer, FOXP1 acts as a tumor suppressor, as loss of nuclear FOXP1 expression and cytoplasmic mislocalization are associated with a poor prognosis, and increased immunoreactivity of nuclear FOXP1 is correlated with a low tumor grade and the status of the hormone receptors, ER-α and progesterone receptor (14–16).