Despite protein-protein interactions having been described previously for MDM2-ERα, MDM2-MDM4, MDM2-p53, MDM4-p53 and ERα-p53 [18, 21, 23, 27, 28, 41, 43, 66, 69, 102–108], our study is the first to describe an interaction between ERα and MDM4 and to identify protein complexes comprised of endogenously-expressed ERα and MDM2 in breast cancer cells. The gene discussed is MDM2; the disease is breast cancer.