ESR1 and breast cancer: Our analysis of fulvestrant resistant BC models led to the identification of an ERα protein isoform (ERα-LBD, 37.3 kDa MW) lacking the N-terminal transcriptional activation (AF1) and DNA binding domains (DBD) but including a portion of the hinge domain followed by the C-terminal domains LBD (Ligand Binding Domain) and AF2.