Vector constructs. Vectors were constructed so that the LTR drives expression of the reporter gene for eGFP and the SV40 promoter drives expression of the gene for neomycin resistance. The 5′LTR of LeGFPSN is the MoMuLV LTR. The MND LTR is based on the MoMuLV LTR, but was modified by replacing the MoMuLV enhancer by the MPSV enhancer (M), deleting the negative control region (N) and replacing the primer binding site (PBS) of the MoMuLV by the PBS of the endogenous murine retrovirus dl587rev (D). Vectors are otherwise identical.