We thank the groups of P. da Fonseca and S. Scheres (MRC Laboratory of Molecular Biology), H. Saibil and E. Orlova (Birkbeck, University of London), J. Huiskonen (University of Oxford), J. Briggs (EMBL Heidelberg), S. Butcher (University of Helsinki), B. Boettcher (University of Edinburgh) and L. Collinson (The Francis Crick Institute) for their help with testing EMPIAR during its development. This work was supported by the UK Medical Research Council with cofunding from the UK Biotechnology and Biological Sciences Research Council (BBSRC; grant MR/L007835 to G.J.K.) and by the BBSRC (grant BB/M018423/1 to J. Swedlow, University of Dundee, Dundee, Scotland). Work on EMDB at PDBe is supported by the Wellcome Trust (grants 088944 and 104948 to G.J.K.), the European Commission Framework 7 Programme (grant 284209 to J. Thornton, EBI, Cambridge, UK), the US National Institutes of Health National Institute of General Medical Sciences (grant R01 GM079429 to W. Chiu, Baylor College of Medicine, Houston, Texas, USA) and EMBL-EBI.