Isolation of putative progenitor cells from primary cultures and cell lines. A. Breast primary cultures were sorted into CALLA single-positive, EPCAM single-positive, double-positive (DP) or double-negative (DN) populations, and expressed as a percentage of total cells. B. TEM analysis revealed a high content of lipofuscin bodies in the DN population sorted from a tumour culture (arrows). C. The DN:DP ratio increased in three types of aggressive tumour (high grade, ER-negative or HER2-positive) relative to non-tumour or non-aggressive tumour cultures. D. The DN:DP ratio in metastatic MDA-MB-231 cells exceeded that in non-tumourogenic MCF-10A cells. E. Activity of the stem cell marker ALDH was similar in non-tumour versus pooled tumour cultures (left), but significantly higher in non-tumour and low grade tumour cultures compared to high grade tumour cultures (p < 0.001; right).