Spheroid culture is a common method to detect stem cell features in vitro [19] . Spheroid colony formation was tested by serial dilution technique. Both CCSP + and CCSP 2 cellular fractions were able to form sphere clones. However, following 10 days of culture ( Figure 5 ) CCSPcells demonstrated a larger colony size and higher efficiency of colony formation than CCSP + cells. Dissociation of spheroid colonies into single cells resulted in reformation of the spheroid colonies, indicating that this phenotype was stable (data not shown).