CD99 (MIC-2 or p30/32) has an unknown function, but has been found to be expressed in vitally all cases of primitive neuroectodermal tumors (PNETs) and Ewing sarcoma.  The specificity is more limited and may be seen in alveolar rhabdomyosarcomas (15%), ALLs (90%), neuroendocrine carcinomas (20%), melanomas, etc.  Therefore, CD99 should not be interpreted alone, and is best used as part of a panel (e.g. AE1/AE3, desmin, S-100, and CD45 in an undifferentiated small round blue cell tumor situation).

 

Note:  CD99 was originally described as being sensitive and “specific” for PNETs.  However, expression in a variety of other tumors (including tumors with similar morphologies to PNETs) were also found to have expression (at least in a subset of cases).  Sensitivity of “new” IHC markers can be determined with a fair degree of accuracy using tissue arrays and large tumor libraries, but it usually takes time with efforts from many different laboratories to fully characterize the “specificity” of a marker.  This should always be taken into consideration when relying heavily on a “new” marker for diagnostic significance.

Wick, MR. “Immunohistochemical approaches to the diagnosis of undifferentiated malignant tumor.”Annals of Diagnostic Pathology12(2008):72-84.

 

Bone Marrow IHC.  Torlakovic, EE, et. al. American Society for Clinical Pathology Pathology Press © 2009.  pp. 123-124.

 

Rao, N., Colby, T. V., Falconieri, G., Cohen, H., Moran, C. A., & Suster, S. (2013). Intrapulmonary solitary fibrous tumors: clinicopathologic and immunohistochemical study of 24 cases. The American Journal of Surgical Pathology, 37(2), 155–166. doi:10.1097/PAS.0b013e31826a92f5