Alternatively, a less time-consuming sucrose density gradient purification method was employed [14] . Clarified supernatants were pelleted twice by ultracentrifugation. Pellets were resuspended in 0.2 M Tris-HCl, pH 7.3, and placed on a discontinuous sucrose density gradient (10-60%) for ultracentrifugation at 100,0009g for 16 h at 4°C. The VLP band, which was visible at the 35% sucrose layer, was collected. Sucrose was removed by dialysis against 1 liter of PBS, and VLPs were concentrated by ultracentrifugation at 100,0009g for 2 h at 4°C.