Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Drosophila polytene chromosomes contain 500 to 1,000 aligned chromatids. The chromosomes shown were isolated from larvae carrying a heat shock protein 70 (hsp 70)-LacZ transgene, in addition to two clusters of endogenous hsp70 genes. Heat shock-induced transcription of the hsp70 genes is accompanied by chromatin decondensation that forms a "puff" Hybridization of chromosomes from the transgenic lines with an hsp70 probe reveals that the hsp70-LacZ transgene produces a puff as large as that produced by the endogenous hsp70 genes. (See related article on p.1470.)