The modification of DNA by 5-methylcytosine (5mC) has essential roles in cell 
differentiation and development through epigenetic gene regulation. 5mC can be 
converted to another modified base, 5-hydroxymethylcytosine (5hmC), by the tet 
methylcytosine dioxygenase (Tet) family of enzymes. Notably, the balance between 
5hmC and 5mC in the genome is linked with cell-differentiation processes such as 
pluripotency and lineage commitment. We have previously reported that the 
maternal factor PGC7 (also known as Dppa3, Stella) is required for the 
maintenance of DNA methylation in early embryogenesis, and protects 5mC from 
conversion to 5hmC in the maternal genome. Here we show that PGC7 protects 5mC 
from Tet3-mediated conversion to 5hmC by binding to maternal chromatin 
containing dimethylated histone H3 lysine 9 (H3K9me2) in mice. In addition, 
imprinted loci that are marked with H3K9me2 in mature sperm are protected by 
PGC7 binding in early embryogenesis. This type of regulatory mechanism could be 
involved in DNA modifications in somatic cells as well as in early embryos.