Although different breast cancer cell lines such as 184A1, SUM149, SUM159, and HCC1954 treated with RA presented a decrease in mammosphere formation [67], the breast cancer MCF-7 cell line responds to RA with an increase of stemness through an ALDH1A1-retinoic acid-HIF-1α-VEGF pathway [26] (Figure 2A). Here, ALDH1A1 is linked to breast cancer.