Hematopoietic progenitor cell antigen CD34 is a leukocyte antigen expressed in various cell types including hematopoietic cells. Antibodies against CD34 are used to identify lymphohematopoietic stem/progenitor cells. CD34 is likely an adhesion molecule with a role in early hematopoiesis by mediating attachment of stem cells to the bone marrow extracellular matrix or directly to stromal cells.