Epithelial stem/progenitor cells are increasingly thought to play a pivotal role in malignancy and are thought to be relatively resistant to conventional cancer therapies. Now, Jeff Rosen's group (p. 468) demonstrate a role for the Wnt/β-catenin pathway – an important developmental signalling mechanism – in mammary epithelial progenitor cell proliferation, self-renewal and radioresistance. They show that the immortalised mammary epithelial cell line COMMA-D-β-geo contains a minor population of Sca+ progenitor-like cells that are capable of self-renewal and asymmetric division. Constitutive activation of β-catenin within these enhances their capacity for self-renewal, which is blocked by inhibition of β-catenin signalling. Furthermore, this Sca+ population sustains significantly less DNA damage and is resistant to clinically relevant doses of radiation. The authors go on to show that active or non-phosphorylated β-catenin levels are higher in the Sca+ cells and that β-catenin relocalises to the nucleus following irradiation, where it upregulates expression of the apoptosis inhibitor survivin. This study provides evidence for progenitor cell radioresistance and defines a novel role for the Wnt/β-catenin pathway in this process. Understanding the mechanisms behind radioresistance in cancer stem cells may be an important factor in improving cancer diagnosis and treatment.