JS-eGFP-mCherry was derived by transiently transfecting 293T cells with (i) a packaging plasmid (pGPP-MX-4CTE) devoid of the JSRV packaging signal (Ψ) and expressing the viral Gag, Pro and Pol; (ii) a plasmid providing the JSRV Env in trans (pC-ML-JSenv, also devoided of Ψ), and (iii) the packaged JSRV vector (pCJS-EFGFP-MC) that upon infection and integration expresses eGFP under the control of an internal promoter (Fig. 7A). The gene discussed is ERVW-1; the disease is infection.