Centromeres are the fragments of DNA that are responsible for proper chromosome 
segregation. They consist of centromeric chromatin surrounded by blocks of 
pericentric heterochromatin, playing an important role in centromere function. 
In somatic cells, the pericentric domains have a specific pattern of epigenetic 
modifications of core histones and contain specific pericentric proteins. These 
features are probably more important for the centromere function than the 
sequence of the centromeric DNA itself. In somatic cells, the HP1alpha and 
HP1beta proteins are indispensable for constitutive heterochromatin formation 
and maintenance. We have analyzed the localization of these proteins in the 
primordial, growing, fully-grown, and maturing mouse oocytes. Additionally, we 
have analyzed post-translational modifications of histone H3, which can 
influence HP1alpha and HP1beta association with the heterochromatin. We showed 
that the regions of constitutive heterochromatin have a distinct pattern of 
histone H3 acetylation and di-, and trimethylation of its lysine 9. We 
demonstrated that HP1beta protein was present in pericentric chromatin domains 
in primordial oocytes, growing (transcriptionally active) oocytes, and in 
fully-grown oocytes, and was released to the cytoplasm after germinal vesicle 
breakdown. In contrast, the HP1alpha was never detected in primordial oocytes, 
was first detected in pericentric heterochromatin in growing oocytes, 
dissociated from pericentric heterochromatin in fully-grown oocytes, and it was 
never detected in maturing oocytes. The presence of HP1alpha and HP1beta 
proteins on the heterochromatin of transcriptionally active oocytes and their 
absence in transcriptionally silent oocytes suggest that they are necessary for 
the repression of RNA synthesis in heterochromatin domains of transcribing 
oocytes.