DNA methylation reprogramming in pre-implantation mouse embryos (fertilisation to approximately E7.5) and the germ lines (approximately E7.5 to puberty). The paternal genome undergoes rapid active genome-wide demethylation immediately after fertilisation, while slower passive demethylation (via DNA replication) affects the maternal genome. Methylation re-establishment occurs at around implantation and it affects both genomes. In the germ line, primordial germ cells lose their DNA methylation during early (black) and late (green) demethylation stages that affect different sequence categories, imprinted regions (permanent gDMRs) being among the late demethylated regions. While methylation is re-established in prenatal male germ cells, in oocytes this process does not complete until after puberty during their final growth stage due to being activated prior to ovulation. The paternal genome is shown in blue while the maternal genome is shown in red. The green lines refer to specific sets of CGIs: CGIs in imprinted regions that maintain allele-specific DNA methylation at least until implantation (permanent gDMRs), the majority of CGIs that maintain their methylation-free state, and CGIs that acquire methylation during oogenesis (gDMRs). The width of the line indicates the relative size of the respective CGI set. The black line represents both parental genomes. PGCs, primordial germ cells; GV, germinal vesicle oocyte; MII, metaphase II oocyte; E, embryonic day.