Figure 8. Schematic model of mammary gland cell populations.

In this model we show the predominant expression of cluster 1 versus cluster 2 risk-TFs with respect to the cell populations found in the mammary gland and the cancer subtypes that arise from them. In the normal mammary gland all three populations have self-renewal capacity. Claudin^low tumours were originally classified as basal in the PAM signature, but are likely to represent a separate lineage arising from myoepithelial cells⁴⁰. Basal-like cancer is thought to arise from alveolar progenitor cells, (The somewhat misleading term ‘basal-like’ reflects the fact these tumours not only express epithelial, but also mesenchymal cell surface markers that are also highly expressed in the myoepithelial lineage located near the basal membrane.) and luminal A/B cancer from ER+ precursors.