Ewing sarcoma is the third most common sarcoma in children and young adults. Its 
characteristic chromosomal rearrangement results in a chimerical EWSR1-ETS 
transcription factor. Secondary genetic alterations are very common. Membranous 
expression of CD99 is seen in almost all tumors. We report 2 unusual cytogenetic 
findings in a pediatric Ewing sarcoma, an insertion of the MIC2 gene encoding 
CD99 from Xp to 10p and a submicroscopic deletion of the well-known tumor 
supressor gene KLF6. The latter has not been described previously in pediatric 
neoplasms. Molecular pathways in tumorigenesis and genetic complexity in cancer 
are discussed.