(A) Mammalian chromosomes generally have several regions where DNA is tightly packed into a structure called heterochromatin (red). These include repeated DNA sequences near to centromeres (called pericentric satellite repeats) and other DNA repeats at the ends of chromosomes (called telomeric DNA repeats). (B) A human Suv39h enzyme called SUV39H1 and two mouse enzymes (Suv39h1 and Suv39h2) all contain a chromodomain (CD; turquoise) and a SET domain (shown in red and yellow), which can add methyl groups to a specific location on histone H3. Suv39h2 also has a basic domain (BD; purple) at the N-terminal end of the protein, while the other two enzymes have a region known as the N-terminal extension (NTE; pink). (C) Johnson et al., Shirai et al., and Velazquez Camacho et al. found that H3K9me3 modifications (small red circles) on histones (blue) and noncoding RNA (green) transcribed from pericentric satellite repeats work together to promote the association of mouse Suv39h1 (left), Suv39h2 (right) and human SUV39H1 (not shown) with heterochromatin. For Suv39h1, different surfaces on the chromodomain are involved in binding to H3K9me3 modifications and RNA, while the NTE interacts with DNA (black) and a downstream factor known as heterochromatin protein 1 (HP1), which is required to silence DNA. For Suv39h2, the basic domain and the chromodomain interact with RNA and H3K9me3, respectively.