Infectivity of MARV particles released into the supernatant of cells 1 to 4 days p.i. was assayed by a 50% tissue culture infective dose (TCID 50 ) assay: Vero cells were grown in 96-well plates to 30 to 40% confluence. Cells were inoculated in quadruplicate with 10-fold serial dilutions of supernatants of HUH-7 cells infected with the MARV Leiden strain for one to four days as described above. The assays were evaluated at 10 days p.i.. TCID 50 values were calculated using the Spearman-Karber method [49] . Equal volumes of each supernatant were separated by SDS-PAGE followed by quantitative immunoblotting on a LiCor Odyssey system using a mouse monoclonal anti-NP antibody, and secondary antibodies, protocols and software (Odyssey version 2.0) provided by the manufacturer. TCID 50 values were normalized to NP levels detected in each supernatant, and the TCID 50 value in the supernatant collected at day 2 p.i. was set to 100%.