In a representative group of BC cell lines including MCF7 and T47D (ER+ hormone sensitive cells correlating with relatively less aggressive and more benign BC characteristics), MDA-MB-231 and MDA-MB-453 (highly invasive and metastasizing ER- cells), compared to normal epithelial breast cells (MCF10A), a histone modification signatures were studied by mass spectrometry (MS) [37]. The gene discussed is ESR1; the disease is breast cancer.