For many years,structure determination of biological macromolecules by cryo-electron microscopy(cryo-EM) was limited to large complexes or low-resolution models.With recent advances in electron detection and image processing,the resolution by cryo-EM is now beginning to rival X-ray crystallography.Now that near-atomic resolution has been consolidated for a range of specimens,the question arises as to where the new size limits lie.The ribosome and virus might be considered easy targets for cryo-EM because they are relatively large,and the latter also has high symmetry.