Here we show that cohesin forms the topological and mechanistic basis for cell-type-specific long-range chromosomal interactions in cis at the developmentally regulated cytokine locus IFNG. Hence, the ability of cohesin to constrain chromosome topology is used not only for the purpose of sister chromatid cohesion, but also to dynamically define the spatial conformation of specific loci. This new aspect of cohesin function is probably important for normal development and disease