The 4H9 (Leu-9) monoclonal antibody specifically recognizes human CD7 which is also known as T-cell leukemia antigen, T-cell surface antigen Leu-9, and LEU-9. CD7 is a ~40 kDa type I transmembrane glycoprotein that has an extracellular region with an N-terminal IgV-like domain followed by an extended O-glycosylated stalk region, a transmembrane region and cytoplasmic tail. The CD7 antigen is expressed throughout T-lymphocyte differentiation. It is present on 85% to 90% of peripheral blood T lymphocytes. In normal individuals, CD7 is expressed on all CD8+ lymphocytes, approximately 90% of CD4+ lymphocytes, and most NK cells. CD7 is weakly expressed on monocytes but not on granulocytes or B lymphocytes. It is expressed on hematopoietic progenitors and 50% of thymocytes. In leukemias, the CD7 antigen is present on the majority of T-lymphoid lineages. CD7 may function in cellular adhesion and play a role in interactions between T cells as well as T cells and B cells.