To estimate the rate of mutation accumulation in normal mammary epithelial cells with ageing, we established 71 single-cell-derived organoids from epithelial cell adhesion molecule (EpCAM)-positive cells isolated from histologically normal mammary tissues in patients with breast cancer or from breast milk provided by healthy volunteers who were breastfeeding (Methods, Extended Data Fig. 1a, Supplementary Note 1 and Supplementary Fig. 1). The gene discussed is EPCAM; the disease is breast carcinoma.