Estrogen Receptor alpha (ERα) is a ligand-inducible transcription factor that mediates estrogen signaling in hormone-responsive breast cancer (BC) and is the primary target of specific anticancer therapies that although effective can generate resistance phenomena that represents a crucial problem in clinical management of patients affected by this disease. DOT1 Like Histone Lysine Methyltransferase (DOT1L) and Menin 1 (MEN1) have been identified as functional component of the ERα mechanism of action. To investigate the involvement of DOT1L and MEN1 in mediating ERα actions in hormone-responsive and endocrine-resistant BC, interaction proteomics was applied to map the DOT1l and MEN1 nuclear interacting partners in MCF7 breast cancer cell nuclei in order to the identifiy new possible therapeutic targets for a more effective pharmacological treatment of endocrine therapy-resistant tumors.