A small fraction of tumor cells are thought to possess the potential for both multiple -lineage differentiation and self-renewal, which underlies the cancer stem cell hypothesis. However, the differentiation mechanisms of these cells have not been elucidated due to a lack of appropriate culture methods. Here, we established a culture condition for maintaining multipotent tumor cells from rat breast tumors using 4 small molecules. Cultured tumor cells in this condition retained their intrinsic myoepithelial features, expressing p63 and CK14 and vimentin. In a xenograft model, the p63-expressing cells formed epithelial tumors containing glandular, squamous and sebaceous compartments. Upon withdrawal of the small molecules, p63 and CK14 expression was lost, with concurrent increase in expression of mesenchymal markers. These transited cells acquired drug resistance and invasiveness and showed massive sarco-matoid tumorigenicity. Epithelial features could not be recovered by re-exposure to the small molecules in the transited cells. Here, we have identified multipotent cancer cells within primary mammary tumors and demonstrated that their plasticity is maintained by the small molecules. (C) 2016 Elsevier Inc. All rights reserved.