PGR and breast carcinoma: Progesterone levels fluctuate (i.e. menstrual cycle, pregnancies, oral contraceptive pill use, hormone replacement therapy) and hence for practical reasons it is impossible to quantify progesterone exposure over time using conventional technologies (i.e. serum analysis); DNAme at those DNA sites which are bound by the progesterone receptor might act as a good surrogate marker for overall exposure and thereby reflects breast cancer predisposition.