PRA expression is mutually exclusive of C/EBPβ, while PRB and C/EBPβ largely co-localize. (A, B) PRA (green) or PRB (green) and C/EBPβ (red) were detected by immunofluorescence in the mammary glands of 13-week old nulliparous mice and 14-day pregnant mice. Nuclei were counterstained with DAPI (blue). Representative tissue sections are shown with the percentages of PR and C/EBPβ co-localization. For 13-week old nulliparous mice (n=2), PRA-C/EBPβ staining: 11 visual fields, 646 cells; PRB-C/EBPβ staining: 9 visual fields, 304 cells. For pregnant mice (n=2), PRB-C/EBPβ staining: 4 visual fields, 482 cells; PRA-C/EBPβ staining: 7 visual fields; 507 cells. Difference between nulliparous PRA-C/EBPβ co-localization and pregnant PRB-C/EBPβ co-localization: p<0.001. 40× magnification; scale bar, 25 mm. (C) The quantities of PR RNA transcribed from the −2494/+769 PR promoter-reporter were measured by qRT-PCR in cells transfected for LIP expression (LIP) in comparison to cells transfected with empty pcDNA3.1 vector (Ctrl). A ratio of PRB to total PR RNA was calculated and these values were normalized to a relative value of 1.0 for cells receiving the empty vector control. The data presented are the means of seven experiments with SEM. p was calculated by Student’s t-Test in comparison to the control transfection. **, p < 0.05.