In the beginning, high ALDH activity enriched in CSCs is exclusively attributed to ALDH1A1, whereas in recent years this high activity has been associated with other isoforms,98 such as ALDH1A399 in breast cancer, ALDH1B1100 in colon cancer, ALDH3A1101 in gastric cancer (GC), and ALDH7A1102 in PCa. Here, ALDH1A1 is linked to breast cancer.