A schematic representation of the viruses used in this study. A-MLV-GFP and GALV-GFP are replication-competent MoMLV in which the MLV envelope (env) gene has been replaced with either A-MLV  or GALV env [14, 15]. Both viruses contain an IRES-GFP cassette between the env gene and 3'LTR. In addition, GALV-GFP also contains an insertion of TCC just upstream of the splice acceptor (SA) resulting in a virus with enhanced infection and replication properties . GALV-GFP-C11D8 is identical to GALV-GFP except that the C11D8 epitope tag (QVMTITPPQAMGPNLVLP) that derives from the amino acid terminus of the FeLV-B proline rich region (PRR) was introduced into the GALV PRR . The relative position of PRR within SU and transmembrane (TM ) subunits of GALV envelope protein is shown. GALV-Gag tomato red was generated by replacing GFP of GALV-GFP with Gag fused in frame to fluorescent tomato red gene in the GALV-GFP plasmid. The retroviral vector plasmid, pRT43.2 βgal contains a CMV immediate early enhancer/promoter in the 5' LTR as well as a β-galactosidase reporter gene.