The term "small round-cell tumor" describes a group of highly aggressive 
malignant tumors composed of relatively small and monotonous undifferentiated 
cells with high nuclear to cytoplasmic ratios. This group includes Ewing's 
sarcoma (ES), peripheral neuroepithelioma (aka, primitive neuroectodermal tumor 
or extraskeletal ES), peripheral neuroblastoma ("classic-type"), 
rhabdomyosarcoma, desmoplastic small round-cell tumor, lymphoma, leukemia, 
small-cell osteosarcoma, small-cell carcinoma (either undifferentiated or 
neuroendocrine), olfactory neuroblastoma, cutaneous neuroendocrine carcinoma 
(aka, Merkel-cell carcinoma), small-cell melanoma, and mesenchymal 
chondrosarcoma. Their clinical presentations often overlap, thus making a 
definitive diagnosis problematic in some cases. Yet, a clear understanding of 
their clinicopathologic features usually allows for a confident diagnosis, 
especially if immunohistochemistry is used. The following is a review of the 
immunohistochemistry of this small round-cell tumor group.