Ewing tumors (ETs) are characterized by rearrangements of the EWS gene located in 22q12 and a high MIC2 expression. In 85% of ETs, the t(11;22)(q24;q12) generates chimeric fusion transcripts between the EWS and the FLI1 gene, whereas in the remaining cases the EWS gene is rearranged with different partners of the ETS oncogene family. Besides classical cytogenetic analysis, fluorescence in situ hybridization (FISH) and RT-PCR can be used to demonstrate these 22q12 rearrangements which are pathognomonic for ETs. To visualize 22q12 rearrangements in individual cells, DNA probes flanking the EWS-R1 breakpoint region on chromosome 22 can be hybridized in double-target FISH experiments on tumor cell preparations. Intact chromosomes 22 are indicated by juxtaposition of the DNA probes, whereas rearrangements of the EWS gene separate the hybridization signals. In addition to 22q12 rearrangements, numerical aberrations of chromosomes 8 and 12 can be observed in about 50% of ETs, deletions at the short arm of chromosome 1 and der (16)t(1;16)(q12;q11.2) chromosomes in about 20% of the cases. Numerical aberrations, deletions at 1p36.3, and the t(1;16) were detected by using double-target FISH on touch, cytospin, and chromosome preparations, on frozen and paraffin sections and isolated deparaffinized nuclei. So far, numerical aberrations of chromosomes 8 and 12 did not show prognostic impact. However, deletions at 1p36.3 and imbalances between the long and short arms of chromosome 1 were associated with adverse clinical outcome in a group of patients with localized disease. Copyright 2000 S. Karger GmbH, Freiburg