Since breast cancer stem cells (BCSCs) were isolated in 2003 and shown to exhibit strong tumourigenicity (Al-Hajj et al., 2003), a large number of studies have described the use of CD44, CD24 and aldehyde dehydrogenase-1 (ALDH1) for the identification of BCSCs (Chiotaki, Polioudaki & Theodoropoulos, 2016; Yang et al., 2017). This evidence concerns the gene ALDH1A1 and breast cancer.