It has been shown for several DNA probes that the recently introduced Fast-FISH (fluorescence in situ hybridization) technique is well suited for quantitative microscopy. For highly repetitive DNA probes the hybridization (renaturation) time and the number of subsequent washing steps were reduced considerably by omitting denaturing chemical agents (e.g., formamide). The appropriate hybridization temperature and time allow a clear discrimination between major and minor binding sites by

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... ve fluorescence microscopy. The welldefined physical conditions for hybridization permit automatization of the procedure, e.g., by a programmable thermal cycler. Here, we present optimized conditions for a commercially available X-specific α-satellite probe. Highly fluorescent major binding sites were obtained for 74 o C hybridization temperature and 60 min hybridization time. They were clearly discriminated from some low fluorescent minor binding sites on metaphase chromosomes as well as in interphase cell nuclei. On average, a total of 3.43 ± 1.59 binding sites were measured in metaphase spreads, and 2.69 ± 1.00 in interphase nuclei. Microwave activation for denaturation and hybridization was tested to accelerate the procedure. The slides with the target material and the hybridization buffer were placed in a standard microwave oven. After denaturation for 20 sec at 900 W, hybridization was performed for 4 min at 90 W. The suitability of a microwave oven for Fast-FISH was confirmed by the application to a chromosome 1-specific α-satellite probe. In this case, denaturation was performed at 630 W for 60 sec and hybridization at 90 W for 5 min. In all cases, the results were analyzed quantitatively and compared to the results obtained by Fast-FISH. The major binding sites were clearly discriminated by their brightness. Correspondence C. Cremer