CD99 antigen also known as MIC2 or single-chain type-1 glycoprotein

Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

 is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the CD99 gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

. The protein has a mass of 32 kD. Although present on the X chromosome

X chromosome

The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...

, unusually, the CD99 gene does not undergo X inactivation, and it was the first such pseudo-autosomal gene

Pseudoautosomal region

The pseudoautosomal regions, PAR1 and PAR2 are homologous sequences of nucleotides on the X and Y chromosomes.The pseudoautosomal regions get their name because any genes located within them are inherited just like any autosomal genes...

 to be discovered in humans.

Expression

It is expressed on all leukocytes but highest on thymocytes and is believed to augment T-cell

T cell

T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...

 adhesion

Cell adhesion

Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...

, apoptosis

Apoptosis

Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

 of double positive t cells, and participates in migration

Cell migration

Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations...

 and activation. There is also experimental evidence that it binds to cyclophilin A

Cyclophilin

Cyclophilins are a family of proteins from vertebrates and other organisms that bind to cyclosporine, an immunosuppressant which is usually used to suppress rejection after internal organ transplants...

.

It is positive in Ewing's sarcoma and granulosa cell tumors. Antibodies to CD99 are used in diagnostic immunohistochemistry

Immunohistochemistry

Immunohistochemistry or IHC refers to the process of detecting antigens in cells of a tissue section by exploiting the principle of antibodies binding specifically to antigens in biological tissues. IHC takes its name from the roots "immuno," in reference to antibodies used in the procedure, and...

 to distinguish Ewing's sarcoma from other tumours of similar histological appearance, as well as for the identification of thymic

Thymus

The thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....

 tumours, and of spindle cell tumours, such as synovial sarcoma

Synovial sarcoma

A synovial sarcoma is a rare form of cancer which usually occurs near to the joints of the arm, neck or leg. It is one of the soft tissue sarcomas....

, haemangiopericytoma, and meningioma

Meningioma

The word meningioma was first used by Harvey Cushing in 1922 to describe a tumor originating from the meninges, the membranous layers surrounding the CNS ....

.