ELF5 and breast cancer: Interestingly, the binding site used by ELF5 to regulate MUC4 expression is the same site used by another Ets factor, PEA3, to regulate MUC4 expression in the pancreas [9], in HC11 mouse mammary epithelial cells and in MAT-B1 and MAT-C1 rat mammary tumour cells [27], suggesting some functional redundancy.