3D electron microscopy of macromolecules is now moving beyond the very successful tomographic approach of using images of a tilt-series of 2D crystals at known angles to single randomly oriented particles with less and less internal symmetry. The success of 3D reconstruction of symmetrical objects was predicated primarily on the redundancy of information due to lateral symmetry in 2D crystals, or to internal rotational symmetry in the icosahedral, helical or cylindrically symmetric particles. Such a spatially predictable redundancy permitted both a reduction in irradiation dose, to reduce radiationinduced structural alterations of the molecules, and an averaging of image noise due to the low number of electrons used and from the specimen support.
For 3D reconstruction from single particles the noise level in the images is crucial for the accurate determination of the relative orientation of the particles. In principle the central line theorem was recognized to be as applicable to asymmetric particles as it was to symmetrical viruses.