ALDH1A1 and breast cancer: We have previously reported that dye-retaining breast cancer cells exhibit de novo chemoresistance and subsequent clonal outgrowth in vitro, as well as elevated activity of the stemness marker and drug metabolising enzyme family aldehyde dehydrogenase (ALDH) [48,49], a member of which (ALDH1A1) was also found to be significantly up-regulated across all breast cancer types here.