We have previously reported significant differences in the expression of ERGs, PAGs and the PRG, RANKL, in ER+ tumours in a retrospective study that related to the major changes in hormone levels that occur during the menstrual cycle.9,10 Here, we have extended that observation in a prospective study to show significant changes of gene expression of ERGs, PRGs and PAGs within individual patients through the menstrual cycle and the degree of variability in these changes between patients. This evidence concerns the gene ESR1 and neoplasm.