Primordial germ cell (PGCs)-specific protein involved in epigenetic chromatin reprogramming in the zygote following fertilization. In zygotes, DNA demethylation occurs selectively in the paternal pronucleus before the first cell division, while the adjacent maternal pronucleus and certain paternally-imprinted loci are protected from this process. Participates in protection of DNA methylation in the maternal pronucleus by preventing conversion of 5mC to 5hmC: specifically recognizes and binds histone H3 dimethylated at 'Lys-9' (H3K9me2) on maternal genome, and protects maternal genome from TET3-mediated conversion to 5hmC and subsequent DNA demethylation. Does not bind paternal chromatin, which is mainly packed into protamine and does not contain much H3K9me2 mark. Also protects imprinted loci that are marked with H3K9me2 in mature sperm from DNA demethylation in early embryogenesis. May be important for the totipotent/pluripotent states continuing through preimplantation development. Also involved in chromatin condensation in oocytogenesis. Localized in the cytoplasm at the primary oocyte stage and in oocytes within mono-laminar follicles. Expressed in the nucleus and cytoplasm of oocytes in bi-laminar and Graafian follicles and during the 2-cell and morula stages. In 3.5 dpc blastocysts localization is mainly nuclear. Mainly localizes in the female pronucleus, localization to the male pronucleus in much weaker. Expressed in the immature oocytes and in newborn ovaries. Subsequently detected in maturing oocytes and in preimplantation embryos. Expressed in pluripotent embryonic but not in differentiated somatic cells. Expressed in blastocysts, epiblasts, primordial germ cells, embryonic gonads and primitive spermatogonia. No expression is detected in adult testes. Detected at 3.5 dpc (at protein level). Activated during the process of germ cell specification at 7 dpc.25, specifically in the founder population of lineage-restricted primordial germ cells (PGCs). Thereafter, expressed in the germ line until about 15.5 dpc in male and 13.5 dpc in female gonads. Expressed during blastocyst, morula and 4-cell embryo stages. Mediates binding to H3K9me2 via N-terminal region, while ability to exclude TET3 from the maternal pronucleus requires the C-terminal part. Null mutation result in apparently normal offspring. No effect on early gonadal PGCs or gross abnormalities in the development of gametes. However, females display severely reduced fertility despite ovulation of normal numbers of oocytes. Null mutation resulted in preimplantation development failure. Embryos rarely reached the blastocyst stage.