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1. Introduction 3082. Electron microscopy 3112.1 Specimen preparation 3112.2 The electron microscope 3112.3 Acceleration voltage, defocus, and the electron gun 3122.4 Magnification and data collection 3133. Digitisation and CTF correction 3173.1 The patchwork densitometer 3183.2 Particle selection 3203.3 Position dependent CTF correction 3213.4 Precision of CTF determination 3214. Single particles and angular reconstitution 3234.1 Preliminary filtering and centring of data 3234.2 Alignments using correlation functions 3244.3 Choice of first reference images 3244.4 Multi-reference alignment of data 3254.5 MSA eigenvector/eigenvalue data compression 3284.6 MSA classification 3304.7 Euler angle determination (‘angular reconstitution’) 3324.8 Sinograms and sinogram correlation functions 3324.9 Exploiting symmetry 3354.10 Three-dimensional reconstruction 3374.11 Euler angles using anchor sets 3394.12 Iterative refinements 3395. Computational hardware/software aspects 3415.1 The (IMAGIC) image processing workstation 3425.2 Operating systems and GUIs 3425.3 Computational logistics 3445.4 Shared memory machines 3445.5 Farming on loosely coupled computers 3465.6 Implementation using MPI protocol 3475.7 Software is what it's all about 3476. Interpretation of results 3486.1 Assessing resolution: the Fourier Shell Correlation 3486.2 Influence of filtering 3516.3 Rendering 3516.4 Searching for known sub-structures 3526.5 Interpretation 3537. Examples 3537.1 Icosahedral symmetry: TBSV at 5·9 Å resolution 3547.2 The D6 symmetrical worm hemoglobin at 13 Å resolution 3567.3 Functional states of the 70S E. coli ribosome 3577.4 The 50S E. coli ribosomal subunit at 7·5 Å resolution 3598. Perspectives 3619. Acknowledgements 36410. References 364In the past few years, electron microscopy (EM) has established itself as an important – still upcoming – technique for studying the structures of large biological macromolecules. EM is a very direct method of structure determination that complements the well-established techniques of X-ray crystallography and NMR spectroscopy. Electron micrographs record images of the object and not just their diffraction patterns and thus the classical ‘phase’ problem of X-ray crystallography does not exist in EM. Modern microscopes may reach resolution levels better than ∼ 1·5 Å, which is more than sufficient to elucidate the polypeptide backbone in proteins directly. X-ray structures at such resolution levels are considered ‘excellent’. The fundamental problem in biological EM is not so much the instrumental resolution of the microscopes, but rather the radiation sensitivity of the biological material one wants to investigate. Information about the specimen is collected in the photographic emulsion with the arrival of individual electrons that have (elastically) interacted with the specimen. However, many electrons will damage the specimen by non-elastic interactions. By the time enough electrons have passed through the object to produce a single good signal-to-noise (SNR) image, the biological sample will have been reduced to ashes. In contrast, stable inorganic specimens in material science often show interpretable details down to the highest possible instrumental resolution.

IMPORTANCE Asthma and wheezing begin early in life, and prenatal vitamin D deficiency has been variably associated with these disorders in offspring. OBJECTIVE To determine whether prenatal vitamin D (cholecalciferol) supplementation can prevent asthma or recurrent wheeze in early childhood. DESIGN, SETTING, AND PARTICIPANTS The Vitamin D Antenatal Asthma Reduction Trial was a randomized, double-blind, placebo-controlled trial conducted in 3 centers across the United States. Enrollment began in October 2009 and completed follow-up in January 2015. Eight hundred eighty-one pregnant women between the ages of 18 and 39 years at high risk of having children with asthma were randomized at 10 to 18 weeks' gestation. Five participants were deemed ineligible shortly after randomization and were discontinued. INTERVENTIONS Four hundred forty women were randomized to receive daily 4000 IU vitamin D plus a prenatal vitamin containing 400 IU vitamin D, and 436 women were randomized to receive a placebo plus a prenatal vitamin containing 400 IU vitamin D. MAIN OUTCOMES AND MEASURES Coprimary outcomes of (1) parental report of physician-diagnosed asthma or recurrent wheezing through 3 years of age and (2) third trimester maternal 25-hydroxyvitamin D levels. RESULTS Eight hundred ten infants were born in the study, and 806 were included in the analyses for the 3-year outcomes. Two hundred eighteen children developed asthma or recurrent wheeze: 98 of 405 (24.3%; 95% CI, 18.7%-28.5%) in the 4400-IU group vs 120 of 401 (30.4%, 95% CI, 25.7%-73.1%) in the 400-IU group (hazard ratio, 0.8; 95% CI, 0.6-1.0; P = .051). Of the women in the 4400-IU group whose blood levels were checked, 289 (74.9%) had 25-hydroxyvitamin D levels of 30 ng/mL or higher by the third trimester of pregnancy compared with 133 of 391 (34.0%) in the 400-IU group (difference, 40.9%; 95% CI, 34.2%-47.5%, P < .001). CONCLUSIONS AND RELEVANCE In pregnant women at risk of having a child with asthma, supplementation with 4400 IU/d of vitamin D compared with 400 IU/d significantly increased vitamin D levels in the women. The incidence of asthma and recurrent wheezing in their children at age 3 years was lower by 6.1%, but this did not meet statistical significance; however, the study may have been underpowered. Longer follow-up of the children is ongoing to determine whether the difference is clinically important.

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