January 14 In this issue, Science begins its “Pathways of Discovery” feature–a yearlong series of monthly essays examining the complex routes by which science and scientists uncover knowledge about the world. In the cover image by artist Michael Gibbs, human figures traverse a mysterious and seemingly endless landscape of winding, intersecting, and splitting pathways. They have covered much territory, but their present positions suggest little about the ultimate lengths and destinations of their journeys. See page 253

February 4 Honeybees that have flown a few meters in a patterned tunnel falsely perceive that they have traveled several hundreds of meters. Experiments that recorded the dances of bees trained to fly through such a tunnel demonstrate that the bee’s “odometer” is visually driven and provide a way of calibrating it. See page 851 [Illustration: Marco Kleinhenz]

February 25 Stem cells can follow a variety of blueprints directing cell differentiation. But the final edifice is not built of stone, and differentiated cells show remarkable plasticity. See the special section on stem cells, beginning on page 1417 , for information about the research, therapeutic implications, and ethical concerns relating to stem cells. [Illustration: Carin Cain]

March 24 This cover is a melding of old and new. The fruit flies are 1934 drawings by E. M. Wallace, which have been superimposed on nucleotide sequence information. They are a reminder of the rich history surrounding Drosophila melanogaster genetics and the new avenues to research that are being opened by the announcement and initial analyses of the Drosophila genome sequence in the special section, beginning on page 2181, and related features. [Drawings are courtesy of the Archives, California Institute of Technology]

March 31 A bag made of 3M-pioneered, multilayer-polymer color mirror is externally illuminated through the front opening with white light. The startling reflective properties, outstanding color tunability, and superb angular performance of this family of thin, flexible mirrors will have applications in signage, optoelectronics, and energy control. See page 2451 [Photo: Don Pitlik, Pitlik Studio]

April 28 Two views of a backbone model of RNA polymerase II, the central enzyme of gene expression, derived from x-ray crystallography. DNA, depicted as a blue helix, was placed in the structure on the basis of results from electron crystallography. The direction of transcription is from right to left in the view at the upper left and from back to front in the view at the lower right. A pink sphere identifies a metal ion at the active center. See page 640 [Image: P. Cramer et al.]

May 5 This radar-derived model of asteroid 216 Kleopatra is color-coded for gravitational slopes (yellow and red, steepest), whose shallowness (orange through purple, blue, green) suggests a surface covered by unconsolidated porous debris. A metallic object the size of New Jersey, Kleopatra may be the remains of a collision of two former pieces of an ancient asteroid’s disrupted core. See page 836 [Graphics: Shigeru Suzuki and Eric DeJong, JPL Digital Image Animation Laboratory]

May 12 Sculpture representing the mechanics of the mammalian circadian clock. Two brass pulleys with oppositely positioned short hands (representing 24-hour positive and negative feedback loops) are belt-driven by the small pulley (transcriptional machinery). The belt connecting the two “loops” also moves the center pulley and its large hand, generating an output rhythm. See page 1013 [Sculpture: Jason S. Reppert; photo: Samuel C. Riley; photo illustration: C. Faber Smith]

May 26 Stages of mitosis in cells of the Indian muntjak, a deer species with only a few chromosomes. This characteristic allows a clear visualization of the condensation (yellow to orange), separation (red to turquoise), and decondensation (green to yellow) of chromosomes throughout mitosis. See the Nuclear Dynamics section, starting on p. 1369 [Fluorescent micrographs: S. Hauf and J.-M. Peters]

July 21 The coaxial cable, long used for demanding electrical applications, has now been used as a model for improving glass optical fibers. A model study demonstrates how light dispersion and polarization shifts that occur in traditional optical fibers can be overcome when light travels through an all-dielectric coaxial cable whose inner and outer walls are made of omnidirectionally reflecting multilayer films, as shown by the model’s electromagnetic fields seen here. See page 415. [Image: M. Ibanescu et al.]

July 28 Murder rates are down in the United States, but violent crime still continues to inflict a heavy toll on society. What preys on the minds of violent individuals, compelling them to harm others? A special section beginning on page 569 probes the biological and psychological roots of violence and reviews the strategies being tested for preventing people from injuring others or themselves. [Illustration: Daniel Mackie/Digital Vision]

August 25 Scanning electron micrograph of a crater that was created during analysis of a melilite crystal from a calcium-aluminum-rich inclusion in the Allende meteorite. This ~40-micrometer-diameter crater was produced by oxygen ion bombardment during secondary ion mass spectrometry. The results indicate that the sample may have contained short-lived radioactive beryllium-10 at the time of its crystallization in the early solar nebula. See page 1334. [Photo: K. D. McKeegan and G. Jarzebinski]

September 8 Chloroflexus aurantiacus, a green nonsulfur bacterium (orange), growing on an agar layer with the cyanobacterium Synechococcus sp. (green). Anoxygenic photosynthetic bacteria, such as Chloroflexus, which do not produce oxygen during the process of photosynthesis, evolved millions of years before oxygenic photosynthetic bacteria, such as Synechococcus, which generate oxygen as a by-product of the photochemical reaction. See page 1724. [Photo: R. Castenholz]

October 27 Dendrites are remarkable branching structures that receive most of the synaptic input to the neuron. A special section starting on page 735 gives an overview of some of the most exciting developments in this field. The image shows a digital reconstruction of the dendritic tree of a cerebellar Purkinje cell (purple, ~140 mm wide) stained with a fluorescent dye, together with an interneuron (green) making synaptic contacts with the Purkinje cell dendrites. [Image: B. Clark and M. Häusser; digital reconstruction, G. Giese using Imaris software]

November 17 Two slices from the visual cortex of the ferret, showing clusters of neuronal axons (green network) extending toward the brain’s surface (green band) (image width, ~1 mm). These ocular dominance columns, previously thought to require activity in the eyes for proper formation, are now shown to form rapidly early in development (bottom) and, at least initially, to be independent of eye input (top). See page 1321. [Image: J. C. Crowley and L. C. Katz]

December 15 Molybdenum dioxide nanowires (300 nanometers in diameter) were electrodeposited from an aqueous solution onto this graphite surface, where each nanowire is formed at a preexisting defect. After reduction in hydrogen gas, metallic molybdenum nanowires are obtained. These nanowires can then be embedded in a polystyrene film and lifted off the surface intact, creating the potential for applications in microelectronics and chemical sensors. See page 2120. [Scanning electron microscope image: M. P. Zach]

December 22 Creatures great and small are having their genomes sequenced, as highlighted in this Breakthrough of the Year special issue. Now that genomics has come of age, the growing library of gene sequence data is opening a new era of sophisticated genetics research. See the Breakthrough of the Year special section, beginning on page 2220, and the Editorial on page 2255. [Illustration: Peter Steiner]