These transcription factors establish a core network that, in cooperation with epigenetic modifiers, non coding RNAs and the c-Myc transcriptional network, orchestrates the pluripotency expression program and suppresses differentiation programs. Recent data suggest that the same core pluripotency factors also play crucial roles in initial cell fate choices. Differentiation signals directly modulate Oct4 and Sox2 protein levels, leading to changes in their genome wide binding profiles and thus initiating lineage selection without prior activation of lineage specification factors. By indexing chromatin states through DNA and histone modification, epigenetic factors ensure stable propagation of transcription programs and thus contribute to cell identity. At the same time, epigenetic marks are typically reversible and the enzymatic systems that set and erase them respond directly or indirectly to environmental signals, providing the necessary plasticity for the dynamic changes of transcription programs 3,4,5-Trimethoxyphenylacetic acid required for progressive differentiation. Although many epigenetic factors and chromatin remodelers have a role in stabilizing the pluripotent ESC state, most are actually not strictly required for its establishment and/or maintenance. This property and the demonstration that ESC self renewal is minimally dependent on extrinsic signaling have led to the concept that the pluripotent state of na? ��ve epiblast and ESCs represents a ground proliferative state relatively independent from epigenetic regulation. In contrast, most epigenetic regulators are required for proper execution of transcriptional programs driving lineage commitment and progression of differentiation. In mammals DNA methylation plays major roles in the control of gene expression 4-(Benzyloxy)phenol during development and differentiation. The importance of DNA methylation for proper development is underscored by the embryonic lethal phenotypes of mice lacking major DNA methyltransferases. These as well as many other studies have established that Dnmt3a and Dnmt3b, together with the catalytically inactive co-factor Dnmt3L, set DNA methylation patterns during embryogenesis and gametogenesis, while Dnmt1 is mainly responsible for maintaining these patterns through cell replication. However, further studies have shown that Dnmt3 enzymes are also required for long term maintenance of DNA methylation patterns and for their dynamic modulation in processes other than development. In embryos with homozygous inactivation of single Dnmt genes as well as in double Dnmt3a and 3b null embryos development arrests well after gastrulation, clearly showing that these enzymes are dispensable for the formation of na? ��ve epiblast. In addition, corresponding Dnmt null ESCs can be readily derived from blastocysts or by direct gene targeting even in the case of triple knockout of all catalytically active Dnmts. Dnmt1 null ESCs have about 20% residual genomic methylation, mostly located in repetitive sequences. TKO ESCs exhibit complete loss of genomic methylation, clearly showing that neither Dnmts nor DNA methylation are required for survival and self renewal of ESCs. Restoring Dnmt1 expression rescues the ability of Dnmt1 null ESCs to form teratomas, contribute to chimeras and complement tetraploid embryos. 
Analogously, the ability to form teratomas is restored in Dnmt3 DKO ESCs upon expression of either Dnmt3 protein. Therefore, functional pluripotency of ESCs is not permanently compromised by the loss of either Dnmt1 or Dnmt3 proteins and consequent loss of DNA methylation.