Cover illustration: The molecular form of the guanosine-5'-phosphate tetramer [Zimmerman (1976). J. Mol. Biol.106, 663-672] has vertices at points of a cubic lattice with parameter a defined by the distance of O6 from the centre. The height is H = 2a and the basis a square scaled by a factor 6 from the central hole. Another square, scaled by 32 and turned by 45°, separates phosphates and bases. The indices of the molecular forms are indicated. Courtesy of A. Janner.
The first direct experimental measurements of structural phase and amplitude in a centrosymmetric crystal from distances in three-beam convergent-beam electron diffraction patterns are presented as a solution to the `phase problem'.