Satellite III DNA has been located by in situ hybridization in chromosomes 1, 
3--5, 7, 9, 10, 13--18, 20--22, and Y and ribosomal DNA (rDNA) in the 
acrocentric chromosomes 13--15, 21, and 22. In the acrocentric chromosomes, the 
satellite DNA is located in the short arm. Here we report comparisons by in situ 
hybridization of the amount of satellite DNA in Robertsonian translocation and 
"normal variant" chromosomes with that in their homologs. In almost all 
dicentric Robertsonian translocations, the amount of satellite DNA is less than 
that in the normal homologs, but it is rarely completely absent, indicating that 
satellite DNA is located between the centromere and the nucleolus organizer 
region (NOR) and that the breakpoints are within the satellite DNA. The amount 
of satellite DNA shows a range of variation in "normal" chromosomes, and this is 
still more extreme in "normal variant" chromosomes, those with large short arm 
(p+ or ph+) generally having more satellite DNA than those with small short arms 
(p- or ph-). The cytological satellites are heterogeneous in DNA content; some 
contain satellite DNA, others apparently do not, and the satellite DNA content 
is not related to the size or intensity of fluorescence of the satellites. The 
significance of these variations for the putative functions of satellite DNA is 
discussed.