The HUH-7 human hepatoma cell line and Vero cells were maintained in Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal calf serum, l-glutamine and penicillin-streptomycin at 37uC under 5% CO 2 . All work with infectious MARV was performed under BSL-4 conditions at the Institute of Virology in Marburg. The MARV Leiden strain, isolated in 2008 in Leiden, the Netherlands [47] , was propagated in Vero E6 cells and purified as described previously [48] . HUH-7 cells were infected with MARV with a multiplicity of infection of approximately 1 plaque-forming unit per cell for one to four days. At the indicated time points, cell culture supernatants were collected and used for viral infectivity assays (see below), or viruses in the supernatants were purified by centrifugation over a sucrose cushion and fixed for 48 hours (h) with 4% paraformaldehyde in PBS for analysis of particle morphology (see below). For EM and ET of infected cells, HUH-7 cells were grown on carbon-coated sapphire disks, infected as above and cell monolayers were fixed at the indicated time points in the cell culture dish with 4% paraformaldehyde/0.1% glutaraldehyde in 0.1M PHEM buffer (60 mM PIPES, 25 mM HEPES, 2 mM MgCl 2 , 10 mM EGTA), pH 6.9 for 30 min, after which the fixative was replaced with 4% paraformaldehyde in 0.1M PHEM. Fixed cells were removed from the BSL-4 lab after 48 h of inactivation.