Background: CD99, also known as MIC2 or single-chain type 1 glycoprotein, is a highly O-glycosylated transmembrane protein with a molecular mass of 32 kDa. CD99 is expressed on the cell membrane of lymphocytes, thymic cortical cells, and ovarian granulosa cells. Most islet cells, testicular Sertoli cells, and some endothelial cells also express CD99. CD99 can be used in the diagnosis of Ewing's sarcoma, primitive neuroectodermal tumor and thymic tumor, and may be helpful in the differential diagnosis of small blue cell tumors.