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.