In eukaryotes, histone methylation is an epigenetic mechanism associated with a 
variety of functions related to gene regulation or genomic stability. Recently 
analyzed H3K9 methyltransferases (HMTases) as SUV39H1, Clr4p, DIM-5, Su(var)3-9 
or SUVH2 are responsible for the establishment of histone H3 lysine 9 
methylation (H3K9me), which is intimately connected with heterochromatinization. 
In this review, available data will be evaluated concerning (1) the phylogenetic 
distribution of H3K9me as heterochromatin-specific histone modification and its 
evolutionary stability in relation to other epigenetic marks, (2) known families 
of H3K9 methyltransferases, (3) their responsibility for the formation of 
constitutive heterochromatin and (4) the evolution of Su(var)3-9-like and 
SUVH-like H3K9 methyltransferases. Compilation and parsimony analysis reveal 
that histone H3K9 methylation is, next to histone deacetylation, the 
evolutionary most stable heterochromatic mark, which is established by at least 
two subfamilies of specialized heterochromatic HMTases in almost all studied 
eukaryotes.