Using three-dimensional organoid culture combined with in vivo mammary ductal tree regeneration assay [24], SNAI2 and Sox9 have been revealed as master regulators in maintaining the normal mammary stem cell status in murine mammary epithelial cells, and ectopic over-expression of SNAI2 together with Sox9 promotes the tumor-initiating ability of human breast cancer cells [18]. The gene discussed is SOX9; the disease is breast cancer.