Of the three different forms of retinoic acid tested, 9-cis-retinoic acid (9cRA) most strongly activated -1797 p27, followed by all-trans-retinoic acid (atRA) and 13-cis-retinoic acid (13cRA) in all three human breast cancer cell lines. The gene discussed is CDKN1B; the disease is breast carcinoma.