To study MMTV RNA packaging, the newly developed in vivo RNA packaging and propagation assay for MMTV was employed that utilizes an MMTV gag/pol expression construct (JA10) for the production of viral structural and enzymatic proteins [33], a heterologous vesicular stomatitis virus glycoprotein (VSV-G) expression vector, MD.G, for the infection of target cells with pseudotyped MMTV particles [34], and MMTV sub-genomic transfer vectors serving as substrates for RNA packaging [33]. Here, ART4 is linked to infection.