CTCF sites are abundant in the genomes of diverse species but their function is 
enigmatic. We used chromosome conformation capture to determine long-range 
interactions among CTCF/cohesin sites over 2 Mb on human chromosome 11 
encompassing the beta-globin locus and flanking olfactory receptor genes. 
Although CTCF occupies these sites in both erythroid K562 cells and fibroblast 
293T cells, the long-range interaction frequencies among the sites are highly 
cell type specific, revealing a more densely clustered organization in the 
absence of globin gene activity. Both CTCF and cohesins are required for the 
cell-type-specific chromatin conformation. Furthermore, loss of the 
organizational loops in K562 cells through reduction of CTCF with shRNA results 
in acquisition of repressive histone marks in the globin locus and reduces 
globin gene expression whereas silent flanking olfactory receptor genes are 
unaffected. These results support a genome-wide role for CTCF/cohesin sites 
through loop formation that both influences transcription and contributes to 
cell-type-specific chromatin organization and function.