CD34 is a highly glycosylated type I transmembrane glycoprotein that serves as a key marker for various stem and progenitor cells, particularly hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). It is selectively expressed on the surface of early-stage HSCs, vascular endothelial cells, and some mesenchymal stem cells. Functionally, CD34 plays a crucial role in mediating cell adhesion and homing of HSCs to the bone marrow through interactions with L-selectin. It also helps maintain the undifferentiated state of progenitor cells and supports their proliferation. In clinical applications, CD34 is widely used to isolate and enrich HSCs for bone marrow transplantation and gene therapy. Additionally, its expression in certain cancers, such as leukemia and some solid tumors, makes it a potential biomarker for cancer diagnosis and a target for therapeutic interventions.