Schematic representation of NGFR-encoding retroviral vectors. The LXSN type of MoMLV vector was used as the parental vector backbone in the construction of LINGFR and the MSCVneoEB vector was used to derive the MINGFR and MINT vectors. In all cases, a NGFR gene has been placed downstream of an IRES, replacing the previousneo genes driven by internal promoters. Inclusion of the IRES potentially allows for coexpression of an upstream gene on bicistronic transcripts which also encode the NGFR reporter. The LTR of MoMLV directs NGFR gene transcription in LINGFR whereas the MSCV LTR is used to express the NGFR gene in MINGFR and MINT. The NGFR gene in MINT was truncated after the transmembrane (TM) domain.