A DA DAPI POSITIVE HUMAN 14P HETEROMORPHISM DEFINED BY FLUORESCENCE IN-SITU HYBRIDIZATION USING CHROMOSOME 15-SPECIFIC PROBES D15Z1 (SATELLITE-III) AND P-TRA-25 (ALPHOID)

We have investigated the use of the satellite III probe, D15Z1, as an alternative to DA/DAPI staining in the identification of chromosome 15 -derived markers. The probe hybridises to the short arm of chromosome 15 under high stringency conditions. We have screened 100 randomly sel ected patients, by fluorescence in-situ hybridisation (FISH), using co -hybridisation experiments with D15Z1 and a whole chromosome library, pBS-15. 88 individuals showed the expected pattern of two D15Z1 signal s on the p-arm of both homologues of chromosome 15, whereas 12 individ uals showed an additional signal on a third acrocentric D-group chromo some. Sequential GTC banding and D15Z1 hybridisation revealed that in each case it was one homologue of chromosome 14 that was D15Z1 positiv e. This pattern always correlated with positive DA/DAPI staining. In c ontrast, the 15 centromere-specific alphoid probe. pTRA-25, gave the e xpected two signals on both homologues of chromosome 15 in every case. Thus, D15Z1 and DA/DAPI signals co-localize while pTRA-25 is 15 centr omere-specific and should be applied for unequivocal identification of chromosome 15-derived markers in clinical studies. The chromosome 14 heteromorphism, as identified by D15Z1, and defined by pTRA-25, may ha ve arisen by intrachromosomal amplification or interchromosomal exchan ge.