Cells were incubated with anti-CBX7 and anti-Cyclin E1 overnight and then with Alexa 488- or 568-labeled anti-mouse or anti-rabbit IgG antibody (Thermo Fisher Scientific, Massachusetts, USA) 2 h at room temperature. After treated with DAPI (Beyotime Biotechnology, Jiangsu, China) for 10 min, cells were examined with Zeiss axiophot photomicroscope (Carl Zeiss AG, Jena, Germany).