Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Rib cage of an 18.5-day-postcoitum Sox11-deficient embryo stained with Alizarin red (bone) and Alcian blue (cartilage) and presented in false color. Attachment of ribs to the sternum follows an irregular pattern, closely corresponding to the asymmetrical distribution of ossification centers along the sternum. Ossification centers are reduced in size and have not fused correctly. (See related article on p. 6635.)