Brominated and iodinated nucleosides are used in X-ray crystallography studies of oligonucleotide structure. They are also photolabile and are used for cross-linking studies to probe the structure of protein-DNA complexes. Antibodies exist to Br-dU and oligonucleotides containing Br-dU can be used as probes. 5-Fluoro-dU can be used as a non-photoreactive alternative to 5-Br-dU with similar electron density. 5-F-dU base pairs more strongly that T to both dA and the dG mismatch. It is also useful for probing DNA structure using 19F NMR spectroscopy.