Expression of Hox genes located on different chromosomes is precisely regulated 
and synchronized during development. In order to test the hypothesis that the 
Hox loci might cluster in nuclear space in order to share regulatory components, 
we performed 3D FISH on cryosections of developing mouse embryos and 
differentiating embryoid bodies. We did not observe any instances of 
co-localization of 4 different Hox alleles. Instances of 2 different alleles 
touching each other were found in 20-47% of nuclei depending on the tissue. The 
frequency of such "kissing" events was not significantly different in cells 
expressing a high proportion of the Hox clusters when compared to cells 
expressing none or only a few Hox genes. We found that the HoxB and HoxC 
clusters, which are located in gene-rich regions, were involved more frequently 
in gene kissing in embryonic nuclei. In the case of HoxB, this observation 
correlated with the positioning of the corresponding chromosome towards the 
interior of the nucleus. Our results indicate that co-regulation of the 
different Hox clusters is not associated with co-localization of the loci at a 
single regulatory compartment and that the chromosomal context may influence the 
extent to which they contact each other in the nucleus.