Disruption of CXCL12/CXCR4 signaling is also a key step in HSC mobilization by G-CSF [34], [35].This lead us to suspect a causative role of CXCR4/CXCL12 in the lymphoid progenitors rebound during the profound lymphopenia observed after allo-HSCT. The gene discussed is CXCR4; the disease is lymphopenia.