Silent information regulator (Sir) proteins are essential for silencing of the mating type loci, telomeres and ribosomal DNA loci in budding yeast, and Sir3 can generate silenced nucleosomes in vitro. Kingston and co-workers now determine the crystal structure of the bromo-associated homology (BAH) domain of Sir3 in complex with the nucleosome core particle at 3.0 Å resolution. BAH interacts extensively with the histone H4 tail and the regions of H3 and H4 that make up the LRS (loss of ribosomal DNA silencing) domain, which are both known to be important for silencing, as well as H2B adjacent to the LRS surface and the H2A–H2B acidic patch. Importantly, many mutations identified in BAH and in the core histones correlate with residues that form physical contacts between histones and BAH, suggesting that the structure reflects biologically important contacts. Assembly of the complex is accompanied by ordering of the H4 tail and conformational changes in BAH. The structure further explains how covalent modification of H4K16 and H3K79 can modulate silencing. Adjacent nucleosomes in the crystal lattice are bridged by dimerization of BAH, and weak self-association of BAH is also seen in solution. So, although this weak interaction is expected to be insufficient to promote nucleosome compaction by itself, it might contribute to compaction in the context of the full-length Sir3 protein. (Science 334, 977–982, 2011)