Tandemly organized simple repetitive sequences are widespread in all eukaryotes. The organization of the simple tetrameric (GACA)n sequences at chromosomal loci has been investigated using in situ hybridization with chemically pure oligonucleotide probes. Both biotin- and digoxigenin-attached (GACA)4 probes reveal specific hybridization signals over the short arms of all acrocentric autosomes in man. In the other examined primates the NOR-bearing autosomes could be detected by in situ hybridization with (GACA)4, and a major concentration of the GACA simple repeats could be observed on the Y chromosome in the gibbon and mouse: the hybridization site in the gibbon Y chromosome coincides particularly with the silver-stainable NOR. In the past, accumulations of (GACA)n sequences were demonstrated mainly on vertebrate sex chromosomes. Therefore, the organization of GACA simple sequences is discussed in the context of their evolutionary potential accumulation and the possible linkage with the primate rDNA loci.

The rare fragile site at 17p12 can be induced in lymphocyte cultures with the AT-specific DNA-ligands distamycin A, DAPI, Hoescht 33258 and berenil. The optimum culture conditions for the experimental induction of fra(17)(p12) were studied. There are indications that fra(17)(p12) is a late-replicating chromosome region in which AT-rich DNA is located. The fragile site also occurs spontaneously in cell cultures of most fra(17)(p12) carriers. A population screening of 250 unselected individuals showed that the frequency of carriers heterozygous for fra(17)(p12) is 2%. The results are compatible with a population being in Hardy-Weinberg equilibrium with respect to fra(17)(p12) and its non-fragile allelomorph. Neither the heterozygous nor the homozygous condition of fra(17)(p12) have any deleterious effects.

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