The mouse sperm genome is 80–90% methylated at its CpG sites in DNA, amounting to about 20 million methylated sites. After fertilization, the paternal chromosome is almost completely demethylated in six hours by an active process, before DNA replication (blue line in Figure). In the mature oocyte, about 40% of its CpG sites are methylated. Demethylation of the maternal chromosome largely takes place by blockage of the methylating enzymes from acting on maternal-origin DNA and by dilution of the methylated maternal DNA during replication (red line in Figure). The morula (at the 16 cell stage), has only a small amount of DNA methylation (black line in Figure). Methylation begins to increase at 3.5 days after fertilization in the blastocyst, and a large wave of methylation then occurs on days 4.5 to 5.5 in the epiblast, going from 12% to 62% methylation, and reaching maximum level after implantation in the uterus. By day seven after fertilization, the newly formed primordial germ cells (PGC) in the implanted embryo segregate from the remaining somatic cells. At this point the PGCs have about the same level of methylation as the somatic cells.