The HCD99 monoclonal antibody recognizes human CD99 also known as CD99 antigen, E2 antigen, MIC2, and T-cell surface glycoprotein E2. CD99 is a type I, single chain transmembrane protein devoid of N-linked glycosylation sites that is encoded by the pseudoautosomal gene MIC2. CD99 has an apparent molecular weight 32 kD and is widely expressed on a variety of tissues. CD99 is highly expressed on thymocytes, T cells, T cell leukemias and lymphomas and is absent on fetal B cells, some B cell lines, eosinophils, granulocytes and the NK-cell line YT. CD99 is involved in spontaneous rosette formation with erythrocytes and may also be involved in other T-cell and hematopoietic cell adhesion pathways. CD99 has been reported to activate a caspase-independent death pathway in T cells under some conditions. CD99 interacts with a number of proteins including ferritin heavy chain 1, karyopherin beta 1, TRIP13, cyclophilin A, annexin II, and ubiquitin-conjugating enzyme E2H. The HCD99 antibody has been reported to be useful for flow cytometric detection of human CD99 and immunohistochemistry (acetone-fixed frozen tissues and formalin-fixed paraffin-embedded tissues).