Metaphase chromosome spreads from human X hamster somatic cell hybrids immunostained with rabbit antibodies (green) to H3 dimethylated at lysine 4 (a) and H3 dimethylated at lysine 9 (b). The inactive X (arrow) is the only human chromosome retained by these cells and is detected with a DNA probe specific for the human X centromere (red). The inactive X retains H3 dimethylated at lysine 9 but lacks H3 dimethylated at lysine 4.