A large fraction of our genome consist of mobile genetic elements.Governing transposons in germ cells is critically important,and failure to do so compromises genome integrity,leading to sterility.In animals,the piRNA pathway is the key to transposon silencing.piRNAs are generally 24-32 nucleotides in length and bind specifically to the PIWI subfamily of Argonaute proteins.Yet the precise molecular details of how piRNAs are formed and how the pathway represses mobile elements remain poorly understood.We know that Rhino,a member of the Heterochromatin Protein(HP1)subfamily of chromo box proteins,is expressed predominantly in the fly ovaries,and mutations in the rhi gene lead to the activation of transposons and female sterility.Rhino consists of two domains: an N-terminal chromo domain and a C-terminal chromo shadow domain.Here we showed the high resolution crystal structure of the chromo domain of Rhino in complex with H3K9me3.The chromo domain forms a dimer which is unique among all known chromo domains and we confirmed that it is important for its biological function.