Changes in chromatin structure are thought to occur during the regulation of gene expression. However, relatively little is known about DNA topology in vivo because we lack suitable probes for its analysis. On p. 3797, Kuniharu Matsumoto and Susumu Hirose describe a technique that might remedy this gap in our knowledge, visualising transcription-coupled, unconstrained negative DNA supercoils for the first time in an interphase genome. Their technique relies on the ability of psoralen to intercalate into DNA. When exposed to UV light, psoralen crosslinks opposite DNA strands at a rate that depends on the degree of negative superhelicity in the DNA. Using biotinylated psoralen and fluorescent streptavidin, the authors visualised psoralen binding to Drosophila salivary gland DNA. Signals indicating unconstrained negative supercoils were visible on many but not all interbands and puffs, sites of active transcription in polytene chromosomes. Inhibition of transcription or nicking of chromatin DNA abolished psoralen crosslinking. Application of this technique to other interphase chromosomes should greatly increase our understanding of DNA topology in vivo.