We report a new subfamily of alpha satellite DNA (pTRA-2) which is found on all 
the human acrocentric chromosomes. The alphoid nature of the cloned DNA was 
established by partial sequencing. Southern analysis of restriction 
enzyme-digested DNA fragments from mouse/human hybrid cells containing only 
human chromosome 21 showed that the predominant higher-order repeating unit for 
pTRA-2 is a 3.9 kb structure. Analysis of a "consensus" in situ hybridisation 
profile derived from 13 normal individuals revealed the localisation of 73% of 
all centromeric autoradiographic grains over the five acrocentric chromosomes, 
with the following distribution: 20.4%, 21.5%, 17.1%, 7.3% and 6.5% on 
chromosomes 13, 14, 21, 15 and 22 respectively. An average of 1.4% of grains was 
found on the centromere of each of the remaining 19 nonacrocentric chromosomes. 
These results indicate the presence of a common subfamily of alpha satellite DNA 
on the five acrocentric chromosomes and suggest an evolutionary process 
consistent with recombination exchange of sequences between the nonhomologues. 
The results further suggests that such exchanges are more selective for 
chromosomes 13, 14 and 21 than for chromosomes 15 and 22. The possible role of 
centromeric alpha satellite DNA in the aetiology of 13q14q and 14q21q 
Robertsonian translocations involving the common and nonrandom association of 
chromosomes 13 and 14, and 14 and 21 is discussed.