We show that methylated lysine 9 of histone H3 (Me9H3) is a marker of 
heterochromatin in divergent animal species. It localises to both constitutive 
and facultative heterochromatin and replicates late in S-phase of the cell 
cycle. Significantly, Me9H3 is enriched in the inactive mammalian X chromosome 
(Xi) in female cells, as well as in the XY body during meiosis in the male, and 
forms a G-band pattern along the arms of the autosomes. Me9H3 is a constituent 
of imprinted chromosomes that are repressed. The paternal and maternal pronuclei 
in one-cell mouse embryos show a striking non-equivalence in Me9H3: the paternal 
pronucleus contains no immunocytologically detectable Me9H3. The levels of Me9H3 
on the parental chromosomes only become equivalent after the two-cell stage. 
Finally, we provide evidence that Me9H3 is neither necessary nor sufficient for 
localisation of heterochromatin protein 1 (HP1) to chromosomal DNA.