Mock and EnvA/VLP incubated HEK cells (non-permissive for EnvA-VLP) were completely negative for β-lactamase activity (green), because EnvA, the glycoprotein of avian leucosis and sarcoma virus A, requires the cognate receptor Tva (which is absent in HEK cells) for viral entry [33,34]. Here, ART4 is linked to sarcoma.