Cohesin is a DNA-binding protein complex that is essential for sister chromatid 
cohesion and facilitates the repair of damaged DNA. In addition, cohesin has 
important roles in regulating gene expression, but the molecular mechanisms of 
this function are poorly understood. Recent experiments have revealed that 
cohesin binds to the same sites in mammalian genomes as the zinc finger 
transcription factor CTCF. At a few loci CTCF has been shown to function as an 
enhancer-blocking transcriptional insulator, and recent observations indicate 
that this function depends on cohesin. Here we review what is known about the 
roles of cohesin and CTCF in regulating gene expression in mammalian cells, and 
we discuss how cohesin might mediate the insulator function of CTCF.