Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Immunostaining of ATM (upper right) and SCP3 protein (lower right) in an undisrupted mouse spermatocyte I nucleus. DNA (lower left) is stained with DAPI. The merged image (upper left) demonstrates that ATM, an important phosphatidylinositol kinase, is dispersed throughout pachytene chromatin and does not bind preferentially to synaptonemal complexes. (See related article on page 7773.)