Twenty years after the initial description of the CD34 antigen, scientists are still interested in elucidating the exact function of a prototypic > in human, primate and mouse models. While this question remains largely unresolved, this has not precluded the development of medical tools using the detection of CD34 in several applications that range from diagnosis of malignant blood diseases through the biological monitoring of haematopoietic cell collection for autologous or allogeneic transplantation. This review focuses on major aspects of CD34 biology.