Epigenetic modification of DNA, mostly methylation, is the way which ensures lineage specity in mammalian organization and propagation of cell types. It is inherited while cells multiply.
In Nature this week (doi:10.1038/nature13648) Reik and Kelsey describe two article ( et al. Nature 511, 606–610 (2014) and et al. Nature 511, 611–615 (2014)) where the methylation patterns in egg, sperm, fertilized eggs and blastocysts are analyzed. As has been found in mice before, blastocysts lose most of their methylation. Later in development the DNA gets remethylated again. This has been suggested but formal proof was lacking. The imprinting – methylated gene regions due to maternal or paternal origin – is not as much removed for maternal genes, but for paternal ones.
Whether these papers will improve human stem cell research is to be seen.