CD34 is a key regulator of hematopoietic stem cell trafficking to bone marrow and mast cell progenitor trafficking in the periphery.

CD34 is a cell-surface sialomucin widely used for hematopoietic stem cell purification and as a marker of most vascular endothelial cells, including those of capillaries in the majority of tissues. Surprisingly, despite extensive research, the function of this sialomucin has remained elusive, with proposed roles ranging from enhancing proliferation or inhibiting differentiation to acting as a proadhesive L-selectin ligand. Here, we review our recent studies, which suggest that CD34 does, indeed, play a role in leukocyte and HSC trafficking, but that this is through its action as a regulated blocker of cell adhesion and enhancer of migration.