Human Cluster of Differentiation 99 (CD99) Protein

Background

CD99, also known as MIC2, is a cell surface glycoprotein expressed on various cell types, including leukocytes, endothelial cells, and certain tumor cells. The CD99 protein plays a role in multiple biological processes, such as cell adhesion, migration, apoptosis, and immune regulation. CD99 expression is crucial in thymocyte differentiation and T cell activation, as well as in leukocyte extravasation, where immune cells move from blood vessels to sites of inflammation. It is also involved in neural cell adhesion and spermatogenesis, highlighting its diverse functional roles across cell types and tissues. CD99 has garnered attention for its clinical implications, especially in cancer, as it is overexpressed in Ewing sarcoma and other tumors, making it a biomarker of diagnostic and therapeutic interest. Additionally, CD99 is involved in the immune response, where it contributes to the regulation of leukocyte activity and inflammation. The molecule also plays a role in apoptosis, promoting cell death in specific contexts, and has a dual role in tumor biology, both enhancing and inhibiting tumor progression depending on the cancer type and tissue context.