Project Summary The new direct electron detectors (DED) now available for electron microscopes, together with improvements in the available software, has made it possible to determine the structures to about 2.5Å resolution of molecular assemblies, that have a molecular mass greater than about 200kDa, given reasonable homogeneity of the sample. We had a Gatan K2 Summit DED detector installed in July 2015 on our FEI Titan Krios electron microscope. The DED has made it possible to re-examine many earlier results of viruses when complexed with cellular receptors or neutralizing antibodies to attain near atomic resolution. That means we will now be able to see the fundamental chemical bonds that determine the properties of viruses and their complexes with other molecules. It may also be possible to achieve near atomic resolution of large dsDNA viruses. We, therefore, plan to re-examine small icosahedral +RNA picornaviruses and icosahedral ssDNA parvoviruses when complexed with various ligands, as well as large dsDNA viruses. This is an enormous opportunity made possible by the convergence of gaining basic knowledge of virus structure and the vast improvements in electron microscope technology. Our plans include the study of the rhino C and the EV-A71 and EV-D68 picornaviruses. The former viruses are responsible for about 50% of asthmatic attacks in children and the latter viruses have been responsible for recent outbreaks in Southeast Asia of hand, foot and mouth disease in children and for the childhood respiratory difficulties in the US in 2014, respectively. In particular we plan to study recent isolates of these viruses complexed with potential cellular receptors and with neutralizing antibodies. We are also planning to study virus-like particles of human B19 parvoviruses when complexed with potential cellular receptor molecules. Many very large icosahedral dsDNA viruses have been isolated in the last decade. Previously these viruses had remained undetected and unknown because they were unable to pass through the filters used to differentiate viruses from bacteria. Although usually cultured on amoeba in the laboratory, their natural hosts include humans. They are able to perform most of the functions of a living cell and challenge the definition of a virus. Infection by Paramecium bursaria chlorella virus type 1 (PBCV-1) has been associated with schizophrenia. However, the full impact on human health of these numerous large viruses has yet to be fully established. Imposing the resolution should allow recognition of the minor capsid proteins and, therefore, insight into the assembly and infection processes of these large viruses.