An active role for SOX2 during mammary tumorigenesis is further supported by data collected in breast cancer cell lines, where SOX2 drives cell proliferation and in vivo tumorigenesis, partially by facilitating the G1/S transition and regulating, in concert with β-catenin, the expression of downstream effector genes such as CCND1 [14,15]. This evidence concerns the gene SOX2 and breast carcinoma.