Centromeres attach chromosomes to the spindle during mitosis, thereby
ensuring the equal distribution of chromosomes into daughter cells. Tr
anscriptionally silent heterochromatin of unknown function is associat
ed with centromeres in many organisms. In the fission yeast Schizosacc
haromyces pombe, the silent mating-type loci, centromeres, and telomer
es are assembled into silent heterochromatin-like domains. The Swi6 ch
romodomain protein affects this silencing, and now it is shown that Sw
i6p localizes with these three chromosomal regions. In cells lacking S
wi6p, centromeres lag on the spindle during anaphase and chromosomes a
re lost at high rates. Thus, Swi6p is located at fission yeast centrom
eres and is required for their proper function.