Fig. 3. Changes in the methylation levels of H3/K9 during the first and second cell cycles of mouse embryos. The embryos were collected at 6, 12, 20 and 30 hours after insemination, designated as samples one-cell G1, one-cell G2, two-cell G1 and two-cell G2, respectively, and subjected to immunocytochemistry using the antibody against methylated H3/K9. The intensity of fluorescence was analyzed semi-quantitatively, as described in Materials and methods. The nuclei of diploid parthenogenetic two-cell G2 embryos were used as controls in each experiment, and the averaged value of the fluorescence intensity in the controls was arbitrarily set at 50%. The fluorescence intensity of each sample was expressed relative to this value. Data were accumulated from two independent experiments, and were expressed on the basis of DNA content. The numbers of blastomeres examined were: 14, 53, 90 and 38 for the one-cell G1, one-cell G2, two-cell G1 and two-cell G2 stages, respectively. The columns and bars represent mean±s.e.m.