The glycosylated transmembrane protein CD99 is involved in many essential cellular functions including cell adhesion, migration, cell death, differentiation, and intracellular protein trafficking. The CD99 gene encodes two distinct proteins, type I is 32 kDa and type II is 28 kDa, which are a result of the alternative splicing of the cytoplasmic region. These CD99 isoforms are expressed in a cell-type-specific manner and may have distinct functions. CD99 is overexpressed in several types of sarcomas, lymphomas, gliomas, neuroendocrine tumors, and some breast cancers. In these tumors, CD99 may have oncogenetic functions that promote migration, invasion, and metastasis of tumor cells. However, other neoplasms, carcinomas, and sarcomas have CD99 expression in benign or early-stage tumors, but lower expression in the advanced-stage counterparts. In these tumors, CD99 may have oncosuppressor signaling, and its re-expression can lead to the reversal of malignancy. Thus, CD99 is an important membrane protein involved in many aspects of cell migration and adhesion in normal and diseased cells.
