Breast cancer-linked gene expression. Fluorescence light micrograph of cells from a mammary gland in a human breast, showing a high level of the gene Sox10 (blue). This gene is associated with aggressive breast cancer and higher levels of stem cell activity. Also highlighted here is the progesterone receptor (red), which indicates non-stem-cell-like mammary cells. A protein called cytokeratin-8 (green) shows the development of the gland.