Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): A dorsal view of the lumbosacral region of the spinal column from the skeleton of a mouse heterozygous for Noggin and homozygous mutant for Dan (secreted antagonists of bone morphogenetic protein signaling). The skeletal morphology of these animals is generally normal, except that the last lumbar vertebra is unilaterally converted to a sacral fate (visible as an ectopic articulation to the pelvis on the right-hand side). (See related article on page 636.)