CYP19A1 and breast cancer: Butein can modulate estrogen metabolism and inhibit proliferation of breast cancer cells as an inhibitor of aromatase with an IC50 value of 3.75 μM. Further enzyme kinetic study revealed that butein acted on aromatase with a mixed type of inhibition and the Ki value was 0.32 μM. The cell number increased by 10 nM -testosterone treatment was remarkably reduced by 5 μM butein, and the administration of flutamide could not reverse the effect [77].