CD99, also known as MIC2 or single-chain type-1 glycoprotein, is a heavily O-glycosylated transmembrane protein encoded by the CD99 gene. It is widely expressed in various cell types, including hematopoietic cells, endothelial cells, and many types of cancer cells. This protein plays crucial roles in multiple biological processes such as cell adhesion, migration, differentiation, and apoptosis. In the immune system, CD99 is involved in T-cell activation, maturation, and the regulation of leukocyte transmigration. It also has a significant impact on tumor biology, as it is often overexpressed in cancers like Ewing sarcoma and certain leukemias. Engagement of CD99 by specific antibodies can induce caspase-independent cell death in tumor cells, making it a potential therapeutic target.