Cytogenetic and fluorescence in situ hybridization (FISH) analysis of an aggressive undifferentiated soft tissue sarcoma diagnosed as primitive neuroectodermal tumor (PNET) revealed an insertion ins(4;X)(q31-32;p11p22) as the sole aberration. To identify the molecular genetic consequences, contigs of bacterial artificial chromosomes (BACs) covering Xp11-p22 and 4q31-32 were constructed. The breakpoint in Xp22 was considered unlikely to be of pathogenetic significance, as it was very close to the Xp telomere, a region devoid of known or predicted genes. The breakpoint in Xp11 was mapped within a BAC clone containing BCOR, encoding a BCL6 (B-cell lymphoma 6)-interacting protein that may influence apoptosis, as the only known gene. FISH analysis with three overlapping clones on normal chromosomes 4 disclosed that the insertion of Xp11 material in der(4) was accompanied by a deletion of chromosome 4 material. Only a predicted gene (XM_094074) was shown to be partially included in the deletion. This gene displays a high similarity with the gene encoding the embryonic blastocoelar extracellular matrix (ECM) protein in sea urchin, which is involved in the migration of the primary mesenchyme cells during embryogenesis. Our results suggest that BCOR and/or an ECM-like protein could be involved in the pathogenesis of a subgroup of PNET or PNET-like sarcomas.