Fig. 9. PRLR signaling in mammary glands at late pregnancy. Erbb4^Flox/FloxWap-Cre mice at P18 were either mock injected or injected with PRL at 5 µg/g body weight for 15 minutes. Mammary gland lysates were analyzed by western blot for phospho-STAT5 (A, top panel) and total STAT5 protein (A, bottom panel). In addition, paraffin wax-embedded mammary glands from mock- (B) or PRL (C)-injected Erbb4^Flox/FloxWap-Cre mice at P18 were stained for phospho-STAT5 by immunohistochemistry. Detection of PRL-stimulated STAT5 activation in mammary glands from biparous Erbb4^Flox/FloxWap-Cre mice at P18 (A,C) indicates intact PRLR signaling. Pregnancy was rescued in Prlr^-/- mice by the administration of progesterone. Mammary glands from progesterone-treated Erbb4^Flox/FloxWap-Cre control mice (D) and progesterone-rescued Prlr-deficient mice (E) at P18 were embedded in paraffin wax and stained for phospho-STAT5 by immunohistochemistry. Phospho-STAT5 immunohistochemical staining of progesterone-rescued Prlr-null mammary glands indicates PRLR-independent STAT5 activation at late pregnancy. Scale bar: 50 µm.