CD34 molecule is a cluster of differentiation molecule present on certain cells within the human body. It is a cell surface glycoprotein and functions as a cell-cell adhesion factor. It may also mediate the attachment of stem cells to bone marrow extracellular matrix or directly to stromal cells. As a ~ 110 kDa monomeric cell surface antigen, CD34 is highly glycosylated with nine potential N-linked and numerous potential O-linked glycosylation sites in its extracellular domain. The CD34 protein is a member of a family of single-pass transmembrane sialomucin proteins that show expression on early hematopoietic and vascular-associated tissue. CD34 is also an important adhesion molecule and is required for T cells to enter lymph nodes. It is expressed on lymph node endothelia whereas the L-selectin to which it binds is on the T cell. It was indicated that CD34 is a phosphorylation target for activated PKC, and couples to the hematopoietic adapter protein CrkL, which were involved in CD34 signaling pathways. CD34 is abberantly expressed in many kinds of tumors and is implicated in leukemogenesis.