ALDH1A1 and breast carcinoma: To assess the size of the BCSC pool more directly we used two different marker systems: activity of ALDH1 and a system developed in our laboratory that uses lack of proteasome activity to identify BCSCs [13], [14] and was recently shown to correlate with early recurrence in early stage breast cancer patients (T1/2, N0/1) after radiation therapy [15].