Negative regulation against Mtv-1 and HBRV gag acts at the transcript level. A) The gag ORF was cloned in the multiple cloning site of pIRES-eGFP upstream of the IRES and GFP, and was under the control of the CMV promoter. B) Expression of GFP was visualized by fluorescence microscopy. The "empty" pIRES-eGFP construct was used as a control for GFP expression. C) Gag levels were quantified by pulse labeling and immunoprecipitation with an MMTV anti-CA antibody. Pr77gag, the Gag precursor (77 KDa).