To characterize epithelial colony cells, we expanded individual colonies in vitro and performed immunocytochemistry by using a panel of stem cell and epithelial cell-specific antibodies. We found that the epithelial colony cells expressed Ccsp, Oct4, and SSEA-1 (Fig. 1B) . We detected the expression of Oct4 in the nuclei, whereas SSEA-1 was mainly found on the cell surface and in the cytoplasm of the progenitor epithelial colony cells (Fig. 1B) . The colony cells expressed epithelial specific markers Pan-K, K-18, and occludin ( Fig. 1C) but not the mesenchymal markers CD29, CD44, and CD90 (data not shown). Differentiation potential of progenitor epithelial cells