It has been brought to our attention that there was an inaccuracy in the above article . In the discussion section the authors state that "One of the assumptions underlying mammary transplant studies is that mammary outgrowths at limiting dilution are clonal (14,000–20,000 cells per cleared fat pad)" and they attribute this to reference 21 "Smith GH: Experimental mammary epithelial morphogenesis in an in vivo model: evidence for distinct cellular progenitors of the ductal and lobular phenotype. Breast Cancer Res Treat 1996, 39:21–31" . In the paper by Smith no claim of clonality is made, rather it is stated that it is not known whether the simultaneous development of both lobular and ductal phenotypes indicates cooperative interaction between the two epithelial progenitors or signals the presence of a third progenitor type capable of producing both ductular and lobular committed daughters. It is, therefore, incorrect that the assumption of clonality based on limiting dilution has been attributed to Gilbert Smith.