A 17β-estradiol (E2) is hydrolyzed to 2-hydroxy-E2 (2-OHE2) and 4-hydroxy-E2 (4-OHE2) via cytochrome P450 (CYP) 1A1 and 1B1, respectively. In estrogen target tissues including the mammary gland, ovaries, and uterus, CYP1B1 is highly expressed, and 4-OHE2 is predominantly formed in cancerous tissues. In this study, we investigated the inhibitory effects of chrysoeriol (luteorin-3′-methoxy ether), which is a natural methoxyflavonoid, ...