Isolation of BCSCs (CD44+/CD24−/CD45−) from primary ERα-positive breast cancer followed by next generation sequencing- and microarray-based gene expression profiling clearly demonstrate that PIK3CA and other PI3K pathway genes are overexpressed in this population and the pathway is known to be involved in maintaining cancer stem cells in ER-positive breast cancer (Hardt et al., 2012). The gene discussed is ESR1; the disease is breast cancer.