Identification of markers for the detection of tumor-initiating cells, first in leukemia by the group of John Dick (CD (cluster of differentiation) 34+/CD38−) and then in solid tumors including glioma (CD133+, aldehyde dehydrogenase, ALDH1+), breast (epithelial cell adhesion molecule, EpCAM+/CD44high/CD24low, ALDH1+), colorectal (CD133+, EpCAMhigh/CD44+, ALDH1+), head and neck squamous carcinoma (CD44+), and other cancers as discussed elsewhere [21], paved the way for isolating, enriching, and analyzing these tumorigenic cells in different tumor entities [15,21,22]. Here, ALDH1A1 is linked to leukemia.