Due to the detection of cancer stem cells via the expression of biomarkers, tumour initiating cells have been identified in leukemia (CD34+, CD38−) [21] and in solid tumours including glioma (CD133+, ALDH1+) [22], breast (EpCAM+/CD44high/CD24low, ALDH1+) [23], and head and neck squamous carcinoma (CD44+) [24]. This evidence concerns the gene ALDH1A1 and leukemia.