Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): 5-Bromo-4-chloro-3-indolyl--D-galactopyranoside staining of Six4 heterozygous mutant embryo, showing spatiotemporally regulated expression of Six4 in somites and myotomes. At embryonic day 13.5, Six4 expression in digits becomes evident. (See related article on page 3343.)