English: The mouse sperm genome has 80–90% overall methylation of its CpG sites, about 20 million methylated sites. After fertilization, demethylation of the paternal chromosomes is almost completed in 6 hours by an active process, before DNA replication (blue line). The mature oocyte has about 40% methylation of its CpG sites. Demethylation of the maternal chromosomes largely takes place by blockage of the methylating enzymes from acting on maternal-origin DNA and dilution of the methylated maternal DNA during replication (red line). The morula (at the 16 cell stage), has only a small amount of DNA methylation (black line). Methylation begins to increase at 3.5 days after fertilization in the blastocyst and a large wave of methylation then occurs in days 4.5 to 5.5 in the epiblast, going from from12% to 62% methylation, then 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 the same level of methylation as the somatic cells.^[1][2]