The article describes DNA sequences of mammalian genomes that are longer than 50 
bases, but consist exclusively of G's and A's ('pure GA-sequences'). Although 
their frequency of incidence should be 10(-16) or smaller, the chromosomes of 
human, chimpanzee, dog, cat, rat, and mouse contained many tens of thousands of 
them ubiquitously located along the chromosomes with a species-dependent 
density, reaching sizes of up to 1300 [b]. With the exception of a small number 
of poly-A-, poly-G-, poly-GA-, and poly-GAAA-sequences (combined <0.5%), all 
pure GA-sequences of the mammals tested were unique individuals, contained 
several repeated short GA-containing motifs, and shared a common hexa-nucleotide 
spectrum. At most 2% of the human GA-sequences were transcribed into mRNAs; all 
others were not coding for proteins. Although this could have made them less 
subject to natural selection, they contained many [corrected] times fewer point 
mutations than one should expect from the genome at large. As to the presence of 
other sequences with similarly restricted base contents, there were 
approximately as many pure TC-sequences as pure GA-sequences, but many fewer 
pure AC-, TA, and TG-sequences. There were practically no pure GC-sequences. The 
functions of pure GA-sequences are not known. Supported by a number of 
observations related to heat shock phenomena, the article speculates that they 
serve as genomic sign posts which may help guide polymerases and transcription 
factors to their proper targets, and/or as spatial linkers that help generate 
the 3-dimensional organization of chromatin.