ESR1 and breast cancer: Earlier, X antigen family, member 1 (XAGE-1) was shown to be expressed only in ER-negative breast cancer cell lines (MDA-MB-231, SK-BR-3, and MDA-MB-468 cells), and no expression in ER-positive breast cancer cell lines (ZR-75-1, MCF-7, and BT-474 cells) [16] suggesting that XAGE-1 transcription may be functional through estrogen receptor pathway.