CENP-A is an essential histone H3-like protein that localizes to the centromeric 
region of eukaryotic chromosomes. Heterozygous and homozygous Cenpa-GFP 
fusion-protein mouse mutants, generated through targeted insertion of the green 
fluorescent protein (GFP) gene into the mouse Cenpa gene locus, show specific 
localized fluorescence at all the centromeres. Heterozygous mice are healthy and 
fertile. Cenpa-GFP homozygotes (Cenpag/g) undergo many cell divisions, giving 
rise to up to one million cells that show relatively accurate differentiation 
into distinct mouse embryonic tissues until day 10.5 when significant levels of 
chromosome missegregation, aneuploidy and apoptosis result in death. Cenpag/g 
embryos assemble functional kinetochores that bind to a host of 
centromere-specific structural and mitotic spindle checkpoint proteins (Cenpc, 
BubR1, Mad2 and Zw10). Examination of the nucleosomal phasing of centromeric 
minor and pericentromeric major satellite sequences indicates that the formation 
of Cenpag/g homotypic nucleosomes is not accompanied by any overt alteration to 
the overall size of the monomeric nucleosomal structure or the spacing of these 
structures. This study provides the first example of an essential centromeric 
protein gene variant in which subtle perturbation at the centromeric 
nucleosomal/chromatin level manifests in a significantly delayed lethality when 
compared with Cenpa null mice.