Non-inducible long-term linage tracing shows that Sox9 is expressed in luminal and myoepithelial cells. A non-inducible Sox9-Cre knock-in mouse model was used to assess the contribution of Sox9+ cells to mature lineages. Sox9-Cre mice were crossed with CAG-EGFP mice. Sox9-Cre; CAG-EGFP mammary gland cut sections were stained with anti-GFP (green) and α-SMA (red) antibodies. DAPI staining is blue. (A) GFP staining shows Sox9 expressing cells contribute to both luminal and myoepithelial lineages. (B) α-SMA (red) is used to highlight the myoepithelial cells.