The 6 ml of the suspended virus was layer on top of linear gradient of 40% (w/w) potassium tartrate, 20mM HEPES, pH 7.4 (bottom) to 15% (w/w) glycerol, 20mM HEPES, pH 7.4 (top) and subjected to isopycnic centrifugation in a SW28 rotor (3 hours at 24,000 rpm at 4°C).