Classical cytologic observations suggested that sister chromatids are more strongly attached at heterochromatic regions, and this suggested that the special structure or composition of heterochromatin might favour cohesin recruitment. In fact, it has been shown that Swi6 (the homolog of HP-1 in "S. pombe") binds to methylated Lys 9 of histone H3 and promotes the binding of cohesin to the centromeric repeats in "S. pombe". More recent studies indicate that the RNAi machinery regulates heterochromatin establishment, which in turn recruits cohesin to this region, both in "S. pombe" and in vertebrate cells. However, there must be other mechanisms than heterochromatin to ensure an augmented cohesion at centromeres, because "S. cerevisiae" lacks heterochromatin next to centromeres, but the presence of a functional centromere induces an increase of cohesin association in a contiguous region, spanning 20-50kb.