Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): mad3 is expressed in S-phase cells in the ventricular zone of the mouse neural tube. There is a strong correlation of cells positive for bromodeoxyuridine (BrdU) incorporation (nuclei stained brown with anti-BrdU following a 1-h pulse) with those positive for mad3 expression as determined by in situ hybridization (yellow speckles in the cytoplasm). A transverse section of the neural tube of an 11.5-day-postcoitum mouse embryo is shown. (See related article on page 703.)