(A) Phylogenetic tree showing different clades of the CMT family and the sequence contexts that they methylate. (B) CMT proteins bind H3K9me2, where it methylates target sequences. The histone methyltransferases KYP/SUVH5/SUVH6 in turn bind methylated non-CG sites, where it di-methylates H3K9 residues. In this way, CMT proteins and KYP/SUVH5/SUVH6 establish a positive feedback loop for the establishment and maintenance of regions of constitutive heterochromatin.