Representative results of CXCR4 and CCR5 mRNA expression analyzed by RT-PCR in quiescent HPCs after purification (A) and during in vitro megakaryocytic (Mk), monocytic (Mo), granulocytic (G), or erythroid (E) differentiation (B). In this experiment, HPCs were 97% CD34⁺ as evaluated by flow cytometry and 95% clonogenetic progenitors. Total RNA from CEM cell line (CXCR4) and monocytes (CCR5) and from UT-7 cell line was used as positive and negative controls, respectively. RT-PCR samples were normalized for GAPDH.