is suggested that lineage-specific genes are primed for expression in ES cells but are held in check by opposing histone marks or chromatin state (5). The pluripotency of ES cells is characterized by a specific epigenetic profile where lineage-specific genes may be accessible, but carry repressive H3K27 tri-methylation modifications. H3K27 methylation is functionally important for preventing expression of these genes in ES cells; 2me-H3K4 is a bivalent mark associated with both repressed and active transcription genes, while the further-methylated form 3me-H3K4 is associated with active gene transcription (20). The acetylated H4 is associated with the genes of active transcription. Thus, the presence of these histone marks associating with opposing chromatin states indicates that they are either associated with genes in check to prevent further lineage differentiation, or for the maintenance of NSC renewal. Chromatin-immunoprecipitation-on-chip (ChIP-on-Chip) or ChIP sequencing analysis will allow further identification of the histone mark-associated genes prevailing during the stem cell maintenance.
