ALDH1A1 and cancer: Retinoids play crucial roles in embryonic development,[1] immunity,[2] stem cell differentiation[3] and lipid metabolism,[4] and their deficiency leads to a variety of human disorders.[5] Aldehyde dehydrogenase 1 (ALDH1), as a detoxifying enzyme functioning in converting retinol into retinoic acid,[6] is overexpressed in several drug‐resistant cancer cells.[7]