FLUORESCENCE IN-SITU HYBRIDIZATION IN 6 P ATIENTS WITH CHROMOSOME-18 STRUCTURAL-ABERRATIONS AND IN 7 PATIENTS WITH MARKER CHROMOSOMES

The purpose of the present study was to use the FISH method to establi sh the origin of chromosome aberrations currently unidentificable by r outine banding procedures. It was done in 13 cases with structurally r earranged chromosomes, seven of them with non-satellited marker chromo somes; in two of the latter an isochromosome 18p was identified which was consistent with a clinical picture of a tetrasomy 18p. FISH with c hromosome-specific painting probes showed a deletion 18q in a girl wit h a cytogenetically balanced t(8;18). Two patients with deletions and two with 18 ring chromosomes were studied using a telomeric probe: bot h deletions had telomeric integrity and telomeric material was not pre sent in the 18 rings. In a patient with an abnormal chromosome 18, the FISH analysis confirmed a pericentric inversion. We conclude from the se results that FISH can provide a rapid and unequivocal cytogenetic d iagnosis, which may improve genetic counseling.