The gene encoding the regulator of chromosome condensation (RCC1) was cloned by virtue of its ability to complement the temperature-sensitive phenotype of the hamster cell line tsBN2, which undergoes premature chromosome condensation or arrest in the G1 phase of the cell cycle at non-permissive temperatures [1-2]. RCC1 homologues have been identified in many eukaryotes, including budding and fission yeast. Mutations in the gene affect pre-messenger RNA processing and transport [3,4], mating , initiation of mitosis  and chromatin decondensation , suggesting that RCC1 is important in the control of nucleo-cytoplasmic transport and the cell cycle. Biochemically, RCC1 is a guanine-nucleotide-exchange factor for the nuclear Ras homologue Ran ; it increases the dissociation of Ran-bound GDP by 105-fold . It may also bind to DNA via a protein-protein complex . Here we show that the structure of human RCC1, solved to 1.7-Angstrom resolution by X-ray crystallography, consists of a seven-bladed propeller formed from internal repeats of 51-68 residues per blade. The sequence and structure of the repeats differ from those of WD40-domain proteins, which also form seven-bladed propellers and include the beta-subunits of G proteins. The nature of the structure explains the consequences of a wide range of known mutations. The region of the protein that is involved in guanine-nucleotide exchange is located opposite the region that is thought to be involved in chromosome binding.