[Fluorescence in situ hybridization in 6 patients with alterations of chromosome 18 and in 7 with marker chromosomes].

The purpose of the present study was to use the FISH method to establish the origin of chromosome aberrations currently unidentifiable by routine banding procedures. It was done in 13 cases with structurally rearranged chromosomes, seven of them with non-satellited marker chromosomes; in two of the latter an isochromosome 18p was identified which was consistent with a clinical picture of a tetrasomy 18p. FISH with chromosome-specific painting probes showed a deletion 18q in a girl with a cytogenetically balanced t(8;18). Two patients with deletions and two with 18 ring chromosomes were studied using a telomeric probe: both deletions had telomeric integrity and telomeric material was not present in the 18 rings. In a patient with an abnormal chromosome 18, the FISH analysis confirmed a pericentric inversion. We conclude from these results that FISH can provide a rapid and unequivocal cytogenetic diagnosis, which may improve genetic counseling.