Despite that Robertsonian translocations (ROBs) are the most common chromosomal 
rearrangements in humans (1/1000 individuals), an exact breakpoint and the 
molecular mechanisms leading to their formation are still not well known. This 
is partly due to the fact that Human Genome Project did not provide any map or 
sequence for the acrocentric short arms. The main aim of our studies was to 
narrow the breakpoints in de novo arising and in familial cases of the most 
frequently occurring ROBs, using eight, previously not tested clones derived 
from 21p. Our results from PCR and FISH analysis showed that only the clones 
CR382285, CR382287, and a small fragment of CR382332 are retained in the 
examined ROBs. Moreover, interphase FISH on monochromosomal hybrids verified the 
orientation of studied clones in relation to centromeres of chromosomes 14 and 
21. Given our results, we propose localization of the breakpoints in or nearby 
to clone CR382332. Summarizing, our results allowed to narrow the region where 
the breakpoints are localized and demonstrated that their position could be the 
same in all common ROBs.