After fertilization, the paternal and maternal genomes are demethylated in order to erase their epigenetic signatures and acquire totipotency. There is asymmetry at this point: the male pronucleus undergoes a quick and active demethylation. Meanwhile the female pronucleus is demethylated passively during consecutive cell divisions. The process of DNA demethylation involves base excision repair and likely other DNA-repair-based mechanisms. Despite the global nature of this process, there are certain sequences that avoid it, such as differentially methylated regions (DMRS) associated with imprinted genes, retrotransposons and centromeric heterochromatin. Remethylation is needed again to differentiate the embryo into a complete organism.