Schematic representation of the lentiviral vector constructs. Only the relevant portions of the integrated provirus are depicted. The EF1-α promoter is used to drive expression of either GFP cDNA in the control vector (top panel) or the CXCR4-IRES-GFP bicistronic cassette of the experimental vector (bottom panel). SD indicates splice donor; SA, splice acceptor; pA, polyadenylation signal.