Comment on "Progesterone/RANKL is a major regulatory axis in the human breast".

In vivo menstrual cycle data support the findings by Tanos et al. that progesterone regulates RANKL in an ex vivo microstructure model of the human breast, but dispute the suppression of estradiol on progesterone-stimulated RANKL expression. RANKL responds to progesterone in a three-dimensional organoid culture model under conditions mimicking luteal-phase hormone concentration, suggesting that the microstructure may not be crucial to demonstrate progesterone responsiveness.