To study the assembly and budding of MARV we performed ET on MARV-infected cells. ET allows the different steps of assembly and budding to be visualized in 3D and the dimensions of viral structures to be measured and analysed. Infected cell monolayers were fixed at 1 day p.i., removed from the BSL-4 facility, processed for EM and cut into 300 nm thick sections in their in situ orientation. Dual-axis tilt series of the sections were acquired in the electron microscope, as described in Materials and Methods, and 3D reconstructions were computationally generated. Viral NCs could be readily identified in 3D reconstructions. They were found around viral inclusions, within the cytoplasm, associated with the PM, incorporated into budding viruses, and in released virus particles ( Figure 2 ). We therefore used the NC itself as a convenient marker for identifying virus assembly intermediates.