ALDH1A1 and breast cancer: High tumorigenity in xenograft models is taken as the gold standard for the identification of CSCs or CSLCs, but they can also be identified by various cell surface markers such as CD44high/CD42low (breast cancer), CD133high (glioblastoma) and high aldehyde dehydrogenase 1 (ALDH1) expression or activity (various solid tumors) [4-7].