Cellular morphology of small cell osteosarcoma, an aggressive variant of osteosarcoma, is similar to Ewing sarcoma, but its molecular pathogenesis is largely unknown. We report the case of a 12-year-old girl with multifocal small cell osteosarcoma positive for the Ewing sarcoma breakpoint region 1 (EWSR1)gene rearrangement by interphase fluorescent in situ hybridization and negative forEWSR1-FLI1,EWSR1-ERG,andEWSR1-WT1fusion transcripts by reverse transcriptase PCR. Rapid amplification of cDNA ends revealed exon 6 of the cAMP-responsive element binding protein 3-like 1 gene (CREB3L1, also known as “OASIS,” NM_52854.2) fused in-frame to theEWSR1exon 11, consistent with theEWSR1-CREB3L1fusion transcript expressed in tumor tissue. The corresponding chimeric gene was confirmed by amplification and subsequent sequencing of the genomic breakpoint between introns 11 and 5 ofEWSR1andCREB3L1, respectively. An ˜70 kDa product in the tumor tissue lysate reacted with the CREB3L1 carboxyterminal antibody, consistent with a 656-amino acid predicted chimeric protein. Immunohistochemistry with the same antibody showed signal translocation from the physiologic perinuclear compartment observed in glia and unrelated osteoblasts to nuclei of tumor cells, consistent with the likely function ofEWSR1-CREB3L1as a transcriptional regulator predicted by its structure. This is the first report of a fusion transcript in osteogenic sarcoma; it demonstrates a relation between molecular mechanisms of small cell osteogenic and Ewing sarcomas. The 3′-end partner and the inferred structure ofEWSR1-CREB3L1, however, are different from those of Ewing sarcoma, suggesting different targets of the new oncogene. © 2011 Wiley Periodicals, Inc.