ESR1 and neoplasm: By using Esr1-Cre/MMTV-TVA/Rosa26-tdRFP mice infected with RCAS-Erbb2 (to generate HER2-positive tumors), the authors could demonstrate that ERαpos tumor cells with an overexpression of HER2 have to progressively lose their ERα expression in order to clonally expand and metastasize [48].