We characterized 21 t(13;14) and 3 t(14;21) Robertsonian translocations for the 
presence of DNA derived from the short arms of the translocated acrocentric 
chromosomes and identified their centromeres. Nineteen of these 24 translocation 
carriers were unrelated. Using centromeric alpha-repeat DNA as 
chromosome-specific probe, we found by in situ hybridization that all 24 
translocation chromosomes were dicentric. The chromatin between the two 
centomeres did not stain with silver, and no hybridization signal was detected 
with probes for rDNA or beta-satellite DNA that flank the distal and proximal 
ends of the rDNA region on the short arm of the acrocentrics. By contrast, all 
24 translocation chromosomes gave a distinct hybridization signal when satellite 
III DNA was used as probe. This result strongly suggests that the chromosomal 
rearrangements leading to Robertsonian translocations occur preferentially in 
satellite III DNA. We hypothesize that guanine-rich satellite III repeats may 
promote chromosomal recombination by formation of tetraplex structures. The 
findings localize satellite III DNA to the short arm of the acrocentric 
chromosomes distal to centromeric alpha-repeat DNA and proximal to 
beta-satellite DNA.