Cover photograph (Copyright © 2002, American Society for Microbiology. All Rights Reserved.): Time series of chromatin fibers labeled with green fluorescent protein in living cells. The dispersed large-scale chromatin fibers in the first frame were produced by targeting the estrogen receptor to the chromatin in the absence of hormone. Successive images are various time points following estrogen treatment, showing a dramatic recondensation of chromatin fibers. (See related article on p. 3437.)