Stains & CD markers

Editorial Board Members: Elizabeth Courville, M.D., Borislav A. Alexiev, M.D.
Deputy Editor-in-Chief: Genevieve M. Crane, M.D., Ph.D.
Frido Bruehl, M.D.
Christian M. Schürch, M.D., Ph.D.

Last author update: 1 April 2021
Last staff update: 13 December 2022

Copyright: 2002-2024,, Inc.

PubMed Search: CD99 [TI]

Frido Bruehl, M.D.
Christian M. Schürch, M.D., Ph.D.
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Cite this page: Bruehl F, Schürch CM. CD99. website. Accessed April 14th, 2024.
Definition / general
  • Transmembrane protein with various functions and limited diagnostic utility
  • In the differential diagnosis of small round blue cell tumors
Essential features
  • Exists in 2 isoforms and has variable oncogenic and tumor suppressor functions
  • Positivity for CD99 is not specific but its absence rules out Ewing sarcoma in the workup of a small blue round cell tumor
  • Dot-like staining pattern has been reported in several disease entities but has so far not been proven to be diagnostically useful
Diagrams / tables

Images hosted on other servers:
Oncojanus function of CD99

Oncojanus function of CD99

Clinical features
Uses by pathologists
  • Rule out Ewing sarcoma / PNET (always shows strong membranous CD99 positivity) if the differential diagnosis is desmoplastic small round cell tumor, Ewing-like sarcoma, neuroblastoma, nephroblastoma (Wilms tumor), small cell carcinoma, rhabdomyosarcoma, olfactory neuroblastoma or small cell osteosarcoma (none show strong membranous CD99 positivity)
    • If CD99 is negative, Ewing sarcoma / PNET is unlikely
    • If CD99 is positive, Ewing sarcoma / PNET needs to be confirmed with additional immunohistochemical and adjunctive molecular testing as many other malignant neoplasms, including lymphoblastic leukemias / lymphomas as well as round and spindle cell sarcomas, are usually CD99 positive (Cancer 1991;67:1886)
  • Detect minimal residual disease by flow cytometry in T cell acute lymphoblastic leukemia (CD99 positive) (Leukemia 2004;18:703, Cytometry B Clin Cytom 2018;94:82)
  • May help grade and subtype ependymoma (Medscape J Med 2008;10:41)
  • CD99 (or other immature T cell markers such as TdT and Cd1a) is particularly useful in evaluating mediastinal and other biopsy samples of possible thymic epithelial neoplasms and in the subtyping of these tumors (Am J Surg Pathol 1997;21:936, Diagn Pathol 2007;2:13)
Prognostic factors
Microscopic (histologic) description
Microscopic (histologic) images

Contributed by Frido Bruehl, M.D.

CD99 in Ewing sarcoma

CD99 in T cell acute lymphoblastic leukemia

CD99 in synovial sarcoma

CD99 in CIC-DUX rearranged sarcoma

EWSR1-NFATC2 rearranged sarcoma

EWSR1-NFATC2 rearranged sarcoma

Positive staining - normal
Positive staining - disease
Negative staining
Additional references
Board review style question #1

CD99 immunohistochemistry is helpful in the following diagnostic scenario

  1. Confirm the diagnosis of Ewing sarcoma
  2. Confirm the diagnosis of mesenchymal chondrosarcoma
  3. Rule out the diagnosis of Ewing sarcoma
  4. Rule out the diagnosis of nephroblastoma (Wilms Tumor)
Board review style answer #1
C. Rule out the diagnosis of Ewing sarcoma. CD99 cannot be used to confirm the diagnosis of Ewing sarcoma, as other entities in the differential diagnosis may express CD99. Confirmation of Ewing sarcoma requires demonstration of EWSR1 gene rearrangement.

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Reference: CD99
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