January 10 COVER A human iris, as seen in a slit lamp photograph. Constriction of the pupils in bright light may be due in part to light acting on cells in the retina (other than the rods and cones that mediate vision), as suggested by studies in mice on pages 222 and 245. [Image: Charla Meyer, Peggy Winckowski]

February 7 COVER Fat cells from the human abdomen. A scanning electron micrograph (SEM; magnification 88) shows human adipocytes covered by strands of connective tissue. This special issue tackles the problem of obesity and its daunting challenges, exploring how science can contribute to the solution. 845 [Photo: S. Nishinaga, Photo Researchers]

February 28 COVER Leaves of sea kale have a naturally undulating surface and cannot be flattened without introducing folds. An important control of leaf flatness is precise regulation of the pattern of cell cycle arrest during development, as described on page 1404. [Photo: E. Coen]

March 14 COVER The transition state for the catalytic oxidation of carbon monoxide (CO) by a hydroxyl intermediate over a model platinum/ruthenium surface in solution, as determined by quantum mechanical calculations. This path removes CO from the anode of a proton exchange membrane fuel cell. Platinum, ruthenium, carbon, oxygen, and hydrogen atoms are colored turquoise, yellow, green, red, and white, respectively. 1683 [Image: Matthew Neurock]

April 11 COVER Haplochromis (Pundamilia) nyererei is one of more than 500 endemic cichlid species of Lake Victoria, East Africa. The astounding diversity of cichlids from Lake Victoria and surrounding waters derives from ancestral populations in the more ancient Lake Kivu, as described on page 325. [Photo: Ad Konings, Cichlid Press, El Paso]

April 25 COVER The little hermit hummingbird (Phaethornis longuemareus) at a flower of Heliconia trichocarpa in Costa Rica. Studies of hummingbirds have shown a correspondence between their favored flowers and the length and curvature of their bills. See page 630. [Image: Phil Savoie]

May 2 COVER An artist’s interpretation of the deep Earth. Experiments on the electron spin state of iron at high pressures indicate unexpected processes occurring within Earth’s lowermost mantle. See page 789. [Illustration: Joël Dyon]

May 9 COVER A 3D high-resolution magnetic resonance angiogram of a human foot, showing blood vessels containing MS-325, an investigational MRI contrast agent. As outlined in the special section in this issue, metals, such as the gadolinium in MS-325, have important roles–both positive and negative–in normal cellular biochemistry, the environment, and medicine. 925 [Image: EPIX Medical, Schering AG, and Berlex Laboratories Inc.]

June 20 COVER Unseen matter and energy predominate in the universe, raising some seemingly simplistic yet fundamental questions: Does it clump? Does it swirl? If more is added, will it curl? The answers remain elusive, but exciting advances are highlighted in this issue. [Illustration: Cameron Slayden] 1893

July 18 COVER Normal and developmentally defective inflorescences of Arabidopsis thaliana. The defective flower buds have a disorganized appearance because they express a transgene encoding a viral suppressor of RNA silencing, which interferes with microRNA function. MicroRNAs are required for proper developmental timing in plants and animals. 325 [Images: Zhixin Xie and Kristin Kasschau]

August 1 COVER Model of the lactose permease of Escherichia coli (LacY) in its inward-facing conformation, with a large hydrophilic cavity open to the cytoplasmic surface at the top. The substrate-binding site (bound sugar shown in gold) is at a similar distance from either side of the membrane (blue-gray). LacY catalyzes the concomitant translocation of a galactoside and a proton across the membrane, as described on page 610. [Image: S. Iwata]

August 8 COVER A growing Escherichia coli bacterial cell labeled with a fluorescent membrane stain (blue) and with fluorescent proteins that visualize the replication origin (green) and terminus region (red) of the chromosome. [Image: I. F. Lau] 779

August 29 COVER A liquid crystal domain (~0.3 mm) of the banana-shaped molecule MHOBOW forms undulating stripes of different birefringence each of which, when viewed between crossed polarizers, shows a color specific to the filament structure from which it was grown, on a background of isotropic liquid (black). See page 1204. [Image: M. Nakata]

September 26 COVER The hubs and links of networks can be found in fields as diverse as molecular biology and animal behavior. Viewpoints and Reviews in this special issue consider biology from a “networks” perspective. 1863 [Image: Carin Cain]

October 10 COVER Scanning electron micrograph of a zebrafish inner ear otolith in which the level of the acidic protein Starmaker was reduced. Abnormal morphogenesis of the otolith is caused by uncontrolled growth of calcium carbonate crystals, as described on page 282. [Image: J. Berger]

October 24 COVER A special section explores the promise of genomic medicine and the real-world obstacles that keep it largely out of reach. [Image: Cameron Slayden] 587

November 7 COVER A stone frieze from Persepolis, built in the 5th century B.C.E. as a ceremonial center for the expanding Persian empire and destroyed by Alexander in 330 B.C.E. Artisans incorporated Greek, Egyptian, Lydian, and Persian building traditions into a unique international style. See page 970. [Photo: Shahrokh Razmjou, Iran Bastan Museum]

December 19 COVER Disks represent an aging and expanding universe. Work this year confirmed a bizarre story of how the cosmos was born and what it is made of. Dark energy is the primary ingredient in a universe whose expansion rate and age are now known with unprecedented precision. See the Breakthrough of the Year special section and the accompanying Editorial. 2038. [Image: Cameron Slayden]