High-ALDH activity has been documented in normal hematopoietic stem cells (HSCs) and in malignant progenitors for acute myeloid leukaemia, melanoma, and ovarian, brain, prostate, and breast cancer.21-32 ALDH being intracellular, colorimetric assays of activity, such as ALDEFLUOR, have been used to identify and isolate normal and tumoral human stem cells by flow cytometry.26,33,34 Many isoforms of ALDH can metabolize the ALDEFLUOR substrate, which complicates the identification of cell-specific enzyme subtypes. The gene discussed is LDHA; the disease is breast cancer.