Cover picture: Stretching of an immunoglobulin (Ig) domain of titin, a muscle protein. In order to reproduce and interpret atomic force microscope single-molecule extension experiments, steered molecular dynamics simulations exerted a force between the termini of the domain. After the force exceeded a threshold, the Ig domain rapidly unfolded. The Ig domain is drawn in cartoon representation with its two -sheets presented in different colors and water molecules drawn in line representation. The snapshots show the system at extensions of 10 Å (top left), 17 Å (top right), 150 Å (center), and 285 Å (bottom). The bar at the lower left corner of each snapshot represents 10 Å. This figure was created with VMD (http://www/ks.uiuc.edu/research/VMD). See the article by Lu et al. on page 662.