Post-translational histone modifications, acting alone or in a context-dependent 
manner, influence numerous cellular processes via their regulation of gene 
expression. Monomethylation of histone H3 lysine 27 (K27me1) is a poorly 
understood histone modification. Some reports describe depletion of K27Me1 at 
promoters and transcription start sites (TSS), implying its depletion at TSS is 
necessary for active transcription, while others have associated enrichment of 
H3K27me1 at TSS with increased levels of mRNA expression. Tissue- and 
gene-specific patterns of H3K27me1 enrichment and their correlation with gene 
expression were determined via chromatin immunoprecipitation on chip microarray 
(ChIP-chip) and human mRNA expression array analyses. Results from erythroid 
cells were compared with those in neural and muscle cells. H3K27me1 enrichment 
varied depending on levels of cell-type specific gene expression, with highest 
enrichment over transcriptionally active genes. Over individual genes, the 
highest levels of H3K27me1 enrichment were found over the gene bodies of highly 
expressed genes. In contrast to H3K4me3, which was highly enriched at the TSS of 
actively transcribing genes, H3K27me1 was selectively depleted at the TSS of 
actively transcribed genes. There was markedly decreased to no H3K27me1 
enrichment in genes with low expression. At some locations, H3K27 
monomethylation was also found to be associated with chromatin signatures of 
gene enhancers.