Chromosomes are composed of functionally distinct domains separated by chromatin boundaries. In the August 10 Science, Noma et al. define the boundaries of the heterochromatin domain at the mating-type locus (mat) in fission yeast (Science 2001, 293:1150-1155). They used chromatin immunoprecipitation (ChIP) and PCR analysis to scan the entire mat region (about 47 kilobases). Methylation of lysine 9 of histone H3 and the Swi6 protein were both enriched in a 20kb interval that contains the mat2/3 silent cassettes. The inverted repeats IR-L and IR-R mark the boundaries of this heterochromatin domain. In contrast, methylation of lysine 4 of histone H3 showed an inverse pattern and was excluded from the mat2/3 region. Noma et al. show that deletion of the boundary IR-L and IR-R repeats resulted in spreading of both H3-Lys9 methylation and Swi6 beyond the normal heterochromatin domain (up to 8 kilobases). Thus, distinct H3 methylation patterns define euchromatic (H3-Lys4-Methyl) and heterochromatic (H3-Lys9-Methyl) domains. The boundaries of these domains are dictated by the presence of insulating elements.