In normal breast and in breast cancer, mRNA for RANKL and RANKL protein expression fluctuate together with serum progesterone fluctuation with the highest level in the luteal phase, suggesting that RANKL is a progesterone signaling modulator in normal and malignant breast tissue and a potential biomarker of progesterone activity and its blockade [58]. The gene discussed is TNFSF11; the disease is breast carcinoma.