To first test that ALDH1A3 can generate atRA in tumor cell lines, we took advantage of the finding that the ALDH1A3-negative cell line MCF7 was sensitive to atRA (Figure 1A), and therefore, we ectopically expressed ALDH1A3 along with the related isoforms ALDH1A1 and ALDH1A2 to create stable MCF7 cell lines (Figure 1D). Here, ALDH1A2 is linked to neoplasm.