RARB and breast cancer: It was reported a 52% reduction in the incidence of mammary cancer in animals treated with retinyl acetate [56]. In vitro studies show that retinoids, in particular 9-cis-RA, inhibit the growth of oestrogen receptor- (ER-) positive through blocking cell cycle [57], but not ER-negative human breast cancer cells [25, 58]; ER-negative cells have been demonstrated to express lower RAR-β levels compared to their ER-positive matched cells [59] and they exhibit retinol-induced growth inhibition when transfected with RAR-β [60].