ESR1 and ductal breast carcinoma in situ: Herein, we investigated both the cytoplasmic and nuclear expression of FOXP1 protein in cases of invasive ductal cancer (IDC) or ductal carcinoma in situ (DCIS), as well as in atypical ductal hyperplasia (ADH) and usual ductal hyperplasia (UDH) of the breast, and further analyzed the association of C-FOXP1 expression with ER, calpain II and other clinicopathological parameters in IDC, and also evaluated the prognostic value of C-FOXP1.