CD99 is a 32 kDa transmembrane glycoprotein which is thought to be involved in cell adhesion processes. CD99 was expressed in lymphocytes, thymic cortical cells, ovarian granulosa cells, islet cells, Sertoli cells of testis, immature terminal deoxynucleotidyl transferase (TDT) positive T cells, some bone marrow precursor cells and fibroblasts. At the same time, CD99 was also expressed in vascular endothelial cells, urethral epithelium, some columnar and squamous epithelial cells and ependymal cells. It is mainly used for the diagnosis of Ewing's sarcoma and primitive peripheral nerve endodermal tumor.