ALDH1A1 and breast cancer: In a previous study we have already demonstrated that M13HS hybrid clones, which originated from spontaneous fusion events between M13SV1-EGFP-Neo human breast epithelial cells and HS578T-Hyg human breast cancer cells, possessed cancer stem/ initiating cell-like characteristics such as an increased frequency of ALDH1 positive cells and an increased capability of forming colonies and mammospheres [29].