we report the first crystalstructures of avidin in a complex with two novel fluorescent pyrene derivatives: 1-biotinylpyrene (B9P) and 1-desthiobiotinylpyrene (D9P). The crystalstructures were solved by molecular replacement using the coordinates of avidin molecule as a starting model and the final models of avidin/B9P and avidin/D9P were refined to resolutions of 2.0 Å and 2.1Å, respectively. Our