Ex vivo. patient samples, ionising radiation reprogrammed differentiated breast cancer cells into induced stem cells. They showed increased mammosphere formation and increased tumorigenicity in xenografts. Reprogramming occurred in a polyploid subpopulation of cells, coinciding with re-expression of the transcription factors Oct4, SOX2, Nanog, and Klf4, and could be partially prevented by Notch inhibition. This evidence concerns the gene POU5F1 and breast cancer.