Single light-optical sections showing the distribution of HP1β (red) and centromeres (green). (a) mouse cumulus cell nuclei before transfer into an oocyte. (b-d) naturally fertilized embryos at 26 hphCG (b, late 1-cell), 36 (c, early 2-cell) and 48 hphCG (d, late 2-cell). Note the presence of chromocenters in panels a (* and insert) and d (*); whereas in 1-cell embryos (b), HP1β is essentially enriched at the NPB periphery of the female pronuclei. At that stage, most centromeres are localized at NPBs periphery (e.g. the centromeres underlined by the arrowhead), but some are excluded (arrows). In panels c and d, the dashed line delineates the nuclear halves either enriched in or devoided of chromocenters. Scale bars, 5 μm.