Members of the long-range, low-copy-number repetitive DNA sequence family chAB4 
are located on nine different human chromosome pairs and the Y chromosome, i.e. 
on the short arms of all the acrocentrics. To localize the chAB4 sequences more 
precisely on the acrocentrics, chAB4-specific probes together with rDNA and a 
number of satellite sequences were hybridized to metaphase chromosomes of normal 
probands and of carriers of Robertsonian translocations of the frequent types 
rob(13q14q) and rob(14q21q). The results demonstrate that chAB4 is located on 
both sides of the rDNA on all the acrocentrics; the exact location, however, may 
be chromosome specific. Chromosome 22, most probably, is the only chromosome 
where chAB4 is found in the direct neighbourhood of the centromere. Fluorescence 
in situ hybridization analyses of metaphase chromosomes of carriers of 
rob(21q22q) revealed breakpoint diversity for this rare type of Robertsonian 
translocation chromosome. A direct involvement of chAB4 sequences in 
recombination processes leading to the Robertsonian translocations analysed in 
this study can be excluded.