Nucleosomes, the basic units of chromatin, are involved in transcription 
regulation and DNA replication. Intronless genes, which constitute 3 percent of 
the human genome, differ from intron-containing genes in evolution and function. 
Our analysis reveals that nucleosome positioning shows a distinct pattern in 
intronless and intron-containing genes. The nucleosome occupancy upstream of 
transcription start sites of intronless genes is lower than that of 
intron-containing genes. In contrast, high occupancy and well positioned 
nucleosomes are observed along the gene body of intronless genes, which is 
perfectly consistent with the barrier nucleosome model. Intronless genes have a 
significantly lower expression level than intron-containing genes and most of 
them are not expressed in CD4+ T cell lines and GM12878 cell lines, which 
results from their tissue specificity. However, the highly expressed genes are 
at the same expression level between the two types of genes. The highly 
expressed intronless genes require a higher density of RNA Pol II in an 
elongating state to compensate for the lack of introns. Additionally, 5' and 3' 
nucleosome depleted regions of highly expressed intronless genes are deeper than 
those of highly expressed intron-containing genes.