Although MCF7 was originally thought to be a monoclonal cell line, it was recently discovered as a population of breast cancer cells (mostly ER+) with high levels of molecular heterogeneity (Leung et al., 2010; Pérez-Yépez et al., 2012; Khan et al., 2017; Roden et al., 2018). The gene discussed is ESR1; the disease is breast cancer.