Assessment of electron crystallographic data obtained from two-dimensionalcrystals of biological specimens

Over the past few years, an increasing number of electron crystallographic studies using two-dimensional crystals have shed light on the structure of biologically important macromolecules. Steady progress in the development o f specimen-preparation techniques and image-processing tools enable researc hers to achieve resolutions in the range of 5-10 Angstrom almost routinely. However, reaching near-atomic resolution remains a formidable task that is likely to require several years. Without doubt, this process will become f ar less time-consuming as methods are improved further. However, the immedi ate future is more likely to be dominated by structures solved to an interm ediate level of resolution. Since the reliability of such structures is mor e difficult to assess than that of density maps at near-atomic resolution a nd as the popularity of electron cryo-microscopy increases it becomes more important to define standardized criteria for the evaluation of electron cr ystallographic data. This article discusses some of the relevant issues wit h the aim of stimulating further discussion about the assessment and presen tation of electron crystallographic data.