The CD99 (HI156) antibody recognizes a 32-kDa type-I single chain transmembrane glycoprotein, called E2 antigen which is the MIC2 gene product. CD99 antigen is expressed on most hematological cells, especially at high density on thymocytes, T cells and the cells of T-cell leukemias and lymphomas. CD99 is differentially expressed on early hematopoietic stages, but lacking on fetal B cells and some B cell lines, eosinophils, granulocytes and NK-cell line YT. CD99 antigen is involved in T-cell adhesion processes and hematopoietic adhesion pathways suc as rosette formation.