SOX9 and breast cancer: Since M13SV1-EGFP-Neo human breast epithelial cells exhibit stem-like properties [24], five distinct M13HS hybrid cell clones (M13HS-1, −2, −4, −7, and −8), which were derived from spontaneous cell fusion events between M13SV1-EGFP-Neo cells and human HS578T-Hyg breast cancer cells [15], were analyzed for the expression of SOX9 and SLUG, which cooperatively determine the mammary (cancer) stem cell state [26], and for the luminal and basal markers cytokeratin 8 (CK8) and CK14 [27].