To further characterize secreted DV1 RSPs, we performed sucrose gradient fractionation on RSPs concentrated from supernatant of HeLa-prME cells. Supernatants were ultracentrifuged and RSPs-containing pellets either resuspended in PBS or in 0.5% Triton-X100 containing PBS to solubilize E from lipid membranes, as described [29] . As expected, Triton-X100solubilized E protein and RSP-associated E protein appeared in distinct fractions of a discontinuous 20 to 60% sucrose gradient ( Figure 6 ). The amount of E glycoprotein in each fraction was quantified. E glycoprotein in non-treated sample sedimented in fractions containing 20% to 30% sucrose ( Figure 6 , black bars) whereas in Triton-X100 treated samples, E protein was solubilized and detected in fractions at the top of the gradient (Figure 6 , white bars). These data further confirm that the RSPs formed in HeLa-prME are secreted into culture medium from which they can be easily purified by ultracentrifugation. Moreover, we have obtained preliminary evidence that DV1 RSPs and DV1 viral particles isolated from HeLa-prME and mammalian infected cells, respectively, sediment at similar densities on 10-60% continuous gradient of sucrose (Dr Philippe Despres, Institut Pasteur, personal communication).