Frozen sections of human and rat placentas were fixed in 4% paraformaldehyde for 15 min, followed by permeabilization with 0.2% Triton X-100 for 15 min. After blocking with 10% goat serum, the sections were incubated overnight at 4°C with the following primary antibodies: TP53BP2 (1:500, Abcam, Cambridge, USA), E2F1 (1:100, Abcam, Cambridge, USA), G9a (1:500, Abcam, Cambridge, USA), DNMT1 (1:500, Abcam, Cambridge, USA), H3K9me2 (1:500, Abcam, Cambridge, USA), and CK-7 (1:50, Abcam, Cambridge, USA). The sections were subsequently washed three times with PBS and incubated with fluorescein-conjugated secondary antibodies (1:500, Abcam, Cambridge, USA) for 1 h at 37°C. To visualize the cell nuclei, the sections were stained with 4,6-diamidino-2-phenylindole (DAPI). The fluorescence was observed and imaged via laser confocal microscopy (Zeiss, Jena, Germany).