Monoclonal antibody (MAb) HBA71, which was raised against Ewing's sarcoma cells, 
recognizes a cell-surface glycoprotein, p30/32MIC2, that is encoded by the MIC2 
gene in the pseudoautosomal region of human chromosomes X and Y. This 
immunohistochemical study evaluates the specificity and sensitivity of MAb HBA71 
for tumor diagnosis. Frozen and paraffin-embedded tissues of more than 300 
tumors of diverse histologic type, including more than 100 small round cell 
tumors of childhood and adolescence, were tested with this MAb by the 
avidin-biotin immunoperoxidase procedure. The authors found HBA71 
immunoreactivity in 61 of 63 Ewing's sarcomas studied and 9 of 11 primitive 
neuroectodermal tumors and peripheral neuroepitheliomas. HBA71-negative tumors 
included neuroblastomas (0 of 24), melanomas (0 of 13), an 
esthesioneuroblastoma, small cell osteosarcomas (0 of 2), a malignant 
ectomesenchymoma, desmoplastic SRCT (0 of 5), and medulloblastomas (0 of 5). 
Heterogeneous expression of HBA71 immunostaining was found in some embryonal 
rhabdomyosarcomas (3 of 14) and astrocytomas (4 of 7), and in a few 
neuroendocrine tumors (4 of 26), carcinomas (3 of 94), and lymphomas (6 of 30). 
Because Ewing's sarcomas are consistently HBA71 positive, the authors searched 
for antigen-positive normal cells that may represent precursors for these 
tumors; however, no obvious candidate for the elusive cell of origin for Ewing's 
sarcoma was identified in the normal fetal tissues tested. Their findings 
indicate that HBA71 is a highly restricted cell-surface antigen of Ewing's 
sarcomas and primitive neuroectodermal tumors, and immunohistochemistry 
employing this antibody may be of value in the differential diagnosis of 
selected small round cell tumors in childhood and adolescence.