Although 4HPR can transactivate certain retinoid receptors and RAR antagonists can partially block 4HPR-induced apoptosis (Sun et al, 1999) compared with ATRA, 4HPR binds with low affinity to RAR and demonstrates poor transactivation of RAR/RXR response elements in human breast cancer cells (Sheikh et al, 1995). This evidence concerns the gene RARA and breast cancer.