The variety of tumor-specific cytogenetic and genetic alterations among small round cell tumors (Ewing family of tumors, rhabdomyosarcoma, neuroblastoma, and lymphoma) increases the possibility of genotypic diagnosis of them. In Ewing's sarcoma and related peripheral primitive neuroectodermal tumors, a (11;22)(q24;q12) translocation is associated with hybrid transcripts of the EWS gene with the FLIl gene. In alveolar rhabdomyosarcoma, a (2;13)(q35;qt4) translocation is associated with a chimeric gene between PAX3 and FKHR. Specific genetic alterations of the short arm of chromosome 1 and amplification of the MYCN gene are diagnostically useful in neuroblastomas as the immunoglobulin or T-cell receptor gene rearrangements and chromosome translocations in lymphomas. Thus, cytogenetics and genetics provide an essential adjunct to diagnostic surgical pathology in the case of small round cell tumors, which often present substantial diagnostic challenges. Likewise, in vitro culture studies represent another approach in determining histogenetic origin, novel genes, novel mechanisms of gene dysregulation, and the biological characteristics of small round cell tumors.