ESR1 and breast cancer: Having already established that ERα, MDM4 and MDM2 are all localized in the nuclear compartment of breast cancer cells (Figure 2), we next used size exclusion column chromatography and co-immunoprecipitation (co-IP) assays to assess the ability of ERα to form a protein complex with MDM4, as compared to MDM2 and p53, which served as positive controls.