Early stage progenitors carry in their membrane a receptor (CXCR4). Bone marrow stromal cells produce a factor , SDF1 (stromal derived factor), that tends to retain the progenitors.
Inhibiting the binding of SDF1 to early stage progenitors (bearing membrane antigen CD 34 but not CD 38), increases mobilization of these progenitors, which can then be extracted from peripheral blood by leukapheresis.(extracorporeal removal of leukocytes from the blood). These cells are then used to repopulate bone marrow of patients who have received therapies that are toxic for their bone marrow.
CXCR4 is a chemokine; each C represents a region with cysteine, X without, and R stands for "receptor"